Doctors and researchers regularly perform blood tests to determine the effectiveness of various drugs. The resulting correlations between the test results and patient health have recently become the subject of numerous “process” patents. That these patents have been upheld by the U.S. Court of Appeals for the Federal Circuit represents a dangerous expansion of traditional patent law. This expansion threatens to stifle free markets and infringe on individual liberty. In Mayo v. Prometheus, the Court will address the important question of whether someone can patent the process of observing correlations between blood test results and patient health. The primary legal issue here is whether naturally occurring correlations are patentable as “process” patents simply because the methods used to administer prescription drugs and test blood may involve “transformations” of body chemistry. Cato’s amicus brief, joined by the Reason Foundation and the Competitive Enterprise Institute, argues that these patents are not “processes” as the term was originally understood in the Patent Act of 1952. We liken medical‐diagnostic patents to other abstract‐process patents — such as software and business‐method patents — that have resulted in financial losses for firms and discouraged innovation, and argue that enforcing these patents “will only serve to further slow the economy, retard technological innovation, distort the free market, and place human health at risk.” Moreover, upholding the patents at issue will impermissibly restrict public‐domain activity because the final step in a medical‐diagnostic patent is an entirely mental one that will be violated whenever a doctor performs a previously public‐domain medical test after learning about the patented correlation. Our brief thus closes by arguing that the Court should also consider the profound First Amendment implications in allowing processes whose final step is entirely mental to be patented. “The Court has repeatedly recognized that the First Amendment protects freedom of thought as well as freedom of speech.” Unlike copyrights, patents lack traditional free‐speech safeguards (such as exceptions for “fair use”) and, therefore, the Court should reject medical‐diagnostic patents as impermissibly restricting the freedom of thought.