Proving Trademark Infringement: Likelihood of Confusion and the Sleekcraft Factors
Last updated: November 2026
To prove trademark infringement, a plaintiff must show a valid, protectable mark that it owns, and that the defendant's use of a similar mark in commerce is likely to confuse consumers about source, sponsorship, or affiliation. (15 U.S.C. §§ 1114(1), 1125(a)(1).) In California federal courts, likelihood of confusion is weighed under the eight Sleekcraft factors. Kolmogorov Law, P.C. litigates these claims for Orange County and Los Angeles businesses on both sides of the caption.
Call (909) 235-6116 or contact us online to discuss a trademark infringement claim or an accusation of one.
The three trademark infringement elements
A trademark infringement claim has three parts. The plaintiff owns a valid mark that the law protects. The defendant used the same or a similar mark in commerce, in connection with goods or services, without consent. And that use is likely to cause confusion, mistake, or deception among consumers. (15 U.S.C. § 1114(1).)
For a registered mark, the statute supplies the first element on paper: the certificate is prima facie evidence of validity, ownership, and the exclusive right to use the mark on the listed goods. (15 U.S.C. § 1057(b).) For an unregistered name or logo, the plaintiff proves ownership through priority of use and shows that the mark is distinctive. The confusion test is the same for both. (15 U.S.C. § 1125(a)(1)(A).) Our page on unregistered trademark rights covers that proof in more detail.
The third element decides most cases. The claim exists when a reasonable consumer, seeing the defendant's mark in the marketplace, would likely think the goods come from the plaintiff or carry its approval.
What counts as a protectable mark
Marks sit on a spectrum. Generic terms, the common name of the product, get no protection at all. Descriptive terms, which name a quality or feature, are protected only once customers have come to associate them with one source, what courts call secondary meaning. Suggestive, arbitrary, and fanciful marks are protected from first use. A coined word is at the strong end; a surname or a geographic term is at the weak end.
Proving likelihood of confusion: the eight Sleekcraft factors
In the Ninth Circuit, which covers every federal court in California, likelihood of confusion is measured by the factors listed in AMF Inc. v. Sleekcraft Boats. (AMF Inc. v. Sleekcraft Boats (9th Cir. 1979) 599 F.2d 341, 348-349.) They are:
- Strength of the mark.
- Proximity of the goods or services.
- Similarity of the marks in sight, sound, and meaning.
- Evidence of actual confusion.
- The marketing channels used.
- The type of goods and the degree of care a purchaser is likely to exercise.
- The defendant's intent in selecting the mark.
- The likelihood that the product lines will expand toward each other.
The court said the list is not exhaustive, and no single factor controls. In practice three often carry most of the weight: similarity of the marks, relatedness of the goods, and whether the parties sell through the same channels. Intent is not required, though proof that a defendant picked the name knowing of the plaintiff's mark lets a court infer that confusion was the goal.
One caution. When a defendant uses the plaintiff's mark to refer to the plaintiff's own product, say a repair shop advertising that it services a particular brand, the Ninth Circuit applies a nominative fair use test instead. That defense, and the others, are covered on our trademark defenses page.
How the dispute usually arrives
Three patterns account for most of the trademark calls to this office. A former distributor or licensee keeps using the brand after the relationship ends. A competitor opens nearby with a name that differs by one word, and reviews, calls, and deliveries start going to the wrong place. Or an online seller lists goods under the client's name and the client learns of it when a customer complains about quality.
On the other side, a small business receives a demand letter over a name it has used for a decade, citing a registration that issued last year. Priority, not the certificate, often decides that fight.
Evidence that moves the needle
Dated proof of first use: invoices, ads, website archives, and sales records. Misdirected emails, calls, and reviews, logged with dates. The defendant's own documents on how it picked the name. And sales data for both parties, which drives the analysis on our damages and remedies page.
Deadlines and where the case is filed
The Lanham Act has no statute of limitations of its own. Courts borrow the closest state period and apply laches, so a plaintiff who waits years after learning of the use risks losing relief.
Federal courts have original jurisdiction over Lanham Act claims regardless of the amount in controversy or the citizenship of the parties. (15 U.S.C. § 1121(a).) For Orange County businesses that means the Central District of California at the Ronald Reagan Federal Building in Santa Ana; for Los Angeles, the First Street Courthouse downtown. State claims under the Model State Trademark Law and the Unfair Competition Law can be joined in the same complaint, or filed in Superior Court on their own. (Bus. & Prof. Code, §§ 14245, subd. (a)(1), 17200.)
What we do
We start with the mark itself: how strong it is, who used it first, and what the registration actually covers. Then we build the confusion case factor by factor, because that is how the judge will read it. Where the facts support it, we seek a preliminary injunction early; a plaintiff who shows likely success on the merits gets a rebuttable presumption of irreparable harm. (15 U.S.C. § 1116(a).) When we defend, we attack the weakest element, usually mark strength or the relatedness of the goods, and we look hard at laches and prior use.
What to bring to the first meeting
Your registration certificate or application, if any. Proof of your first use of the mark and continuous use since. Examples of the other side's use, dated. Any confusion evidence. Any letters exchanged. And your sales figures for the period of overlap. If you received a cease and desist letter, bring it unanswered.
Frequently asked questions
How do you prove trademark infringement?
Show that you own a valid mark, that the defendant used a similar mark in commerce without your consent, and that the use is likely to confuse consumers about the source or sponsorship of the goods. Registration supplies prima facie proof of the first element. Confusion is proved through the Sleekcraft factors and, where it exists, evidence of real customers who were misled. (15 U.S.C. §§ 1057(b), 1114(1).)
What is likelihood of confusion?
It is the probability that an appreciable number of reasonably prudent consumers would be confused about the source, sponsorship, or affiliation of goods or services because two marks are similar. Actual confusion helps but is not required. Courts in California weigh the eight Sleekcraft factors listed above.
What are the elements of trademark infringement?
A protectable mark the plaintiff owns; the defendant's unauthorized use of the same or a similar mark in commerce in connection with goods or services; and a likelihood of confusion. For unregistered marks the claim arises under section 43(a) of the Lanham Act, and the plaintiff must also prove the mark is distinctive. (15 U.S.C. § 1125(a)(1)(A).)
Does the Sleekcraft test apply in state court?
California courts deciding claims under the Model State Trademark Law and the Unfair Competition Law generally apply the same likelihood of confusion analysis as the federal courts, and the Sleekcraft factors are routinely cited. The state registered-mark claim tracks the federal statute closely. (Bus. & Prof. Code, § 14245, subd. (a)(1).)
Talk to a California trademark litigation attorney
If a competitor's name is confusing your customers, or you have been accused of confusing someone else's, call (909) 235-6116 or contact us online. Kolmogorov Law, P.C. is at 327 Magnet, Irvine, CA 92618, and represents businesses in Orange County, Los Angeles, and throughout California. See our IP litigation page and our Orange County trademark lawyer page.
This page provides general information about federal and California trademark law and is not legal advice. Outcomes depend on the strength of the mark, the evidence of confusion, and the court.