In January 1952 a team of medical researchers from Cornell Medical College learned that tuberculosis raged untreated on the Navajo Reservation in Arizona. These researchers, led by Walsh McDermott, recognized a valuable opportunity for medical research, and they began a ten-year project to evaluate the efficacy of new antibiotics and test the power of modern medicine to improve the health conditions of an impoverished rural society. The history of this endeavor exposes a series of tensions at the heart of medical research and practice. Researchers exploited the opportunities made possible by the ill-health of a marginalized population, but did so with the cooperation and gratitude of the Navajo. They introduced new antibiotics that liberated patients from hospitals, but erected an intrusive system of outpatient surveillance. They provided innovative health-care services, but failed to reduce the dominant causes of morbidity and mortality. As every act of treatment became an experiment, they risked undermining the trust on which research and clinical care depended.