Responding to a Patent Infringement Claim

1. Note the dates

• The letter. Keep the envelope or email header. The date a business learns of a patent can matter later on willfulness and on damages. A response date set in a letter has no legal force by itself.
• Service of a complaint. A defendant generally has 21 days after service to answer, or 60 days if it waives formal service (Fed. R. Civ. P. 12(a)(1)(A)).
• The one-year clock. A petition for inter partes review must be filed within one year after the business is served with a complaint for infringement of the patent (35 U.S.C. § 315(b)). A letter alone does not start this clock.

2. Preserve evidence

Once litigation is reasonably anticipated, keep documents, source code, product versions, sales records, and messages that bear on the accused product, and suspend routine deletion. Federal courts may sanction the loss of electronically stored information (Fed. R. Civ. P. 37(e)). Texas courts apply Brookshire Brothers, Ltd. v. Aldridge, 438 S.W.3d 9 (Tex. 2014).

3. Get the patent and its history

• The patent and file history. Download both from the USPTO's Patent Public Search and Patent Center. The file history shows what the owner told the examiner, which can narrow the claims.
• Ownership. Check the USPTO Assignment Center to confirm that the sender owns the patent or has the right to enforce it.
• Term and fees. A utility patent generally expires 20 years after its earliest nonprovisional filing date, adjusted for Patent Office delay (35 U.S.C. § 154). Maintenance fees are due 3½, 7½, and 11½ years after grant, and the patent lapses if a fee goes unpaid after the six-month grace period (§ 41(b)). The Maintenance Fees Storefront shows the status.
• Earlier fights. Search PACER for lawsuits on the same patent and the PTAB's P-TACTS system for earlier challenges.

4. Compare the claims with the product

Infringement turns on the claims, element by element. If the product lacks even one element of a claim, that claim is not literally infringed, though the owner may argue that a substitute is equivalent. Compare each asserted claim with the product as it was made and sold during the damages period.

5. Check marking and notice

If the owner sells a patented product without marking it, damages generally start only when the accused party receives actual notice (35 U.S.C. § 287(a)). The letter may be that notice. Damages cannot reach back more than six years before a suit is filed (§ 286).

6. Look at the sender

Is the sender the owner, an exclusive licensee, or an agent? Does the letter identify the patent, the owner, and the accused product? If the business is an end user in Texas, a letter that falsely claims a suit has been filed, asserts an expired or invalidated patent, or omits that basic information in a materially misleading way may violate Tex. Bus. & Com. Code § 17.952. Only the Attorney General can enforce that law (§§ 17.953, 17.955).

7. Turn to suppliers and contracts

• Contracts. Review purchase and supply contracts for indemnity and defense clauses, and give notice as they require.
• Default warranty. Absent a contrary agreement, a merchant seller warrants goods free of rightful infringement claims. A buyer who supplied the specifications must instead hold the seller harmless (Tex. Bus. & Com. Code § 2.312(c)).
• The manufacturer. If the product came from a manufacturer, ask whether it will take over the defense.

8. Notify insurers

Read every policy that might respond, including any IP policy, and give notice promptly. Many commercial general liability policies exclude patent claims, but notice costs little and late notice can forfeit coverage.

9. Keep the analysis privileged

Route technical and legal analysis through counsel. Internal emails that speculate about infringement can become evidence. A business does not have to obtain a lawyer's opinion, and the absence of one cannot be used to prove willful or induced infringement (35 U.S.C. § 298). A sound opinion can still help show good faith.

10. Choose a course

• Ask for specifics. Request the claims at issue and a claim chart.
• Design around. Change the product so it no longer meets a claim element.
• Negotiate. Take a license or a covenant not to sue.
• Challenge the patent. File an inter partes review or request ex parte reexamination.
• Sue first. Bring a declaratory judgment action where there is an actual controversy.
• Wait. Wait while monitoring, with the risks that waiting carries.

The right course depends on the claims, the prior art, the product's revenue, and the cost of each path.

Sources.

• 35 U.S.C. §§ 41(b), 154, 286, 287(a), 298, 315(b)
• Fed. R. Civ. P. 12(a), 37(e)
• Tex. Bus. & Com. Code §§ 2.312(c), 17.952-17.955
• Brookshire Brothers, Ltd. v. Aldridge, 438 S.W.3d 9 (Tex. 2014)

This guide is general information, not legal advice. Law checked through October 20, 2026. See the Disclaimer.