Why do so few patients with cancer enroll in clinical trials? A new @JAMANetworkOpen study of >205,000 patients with advanced cancer, all treated at practices offering trials, found that only ~5% participated. Strikingly, >80% of enrollments came from ~20% of physicians and practices, while ~30% of physicians never enrolled a single patient. Trial access is not just about patient willingness. Where patients are treated, research infrastructure, and physician engagement matter.