Medical students may be at high risk for occupational exposures to blood. To measure the frequency of medical students' exposure to infectious body substances, to identify factors that affect the probability of such exposure, and to suggest targets for the prevention of such exposure. Review of all exposures reported by medical students at the University of California, San Francisco, School of Medicine. Teaching hospitals affiliated with the University of California, San Francisco. Third- and fourth-year medical students from the classes of 1990 through 1996 at the University of California, San Francisco, School of Medicine. A needlestick hotline service was instituted at teaching hospitals affiliated with the University of California, San Francisco, and a required course was created to train students in universal precautions and clinical skills before the beginning of the third-year clerkship. Reports of exposures made to the needlestick hotline service, including type of exposure, training site, clerkship, and time of year. 119 of 1022 medical students sustained 129 exposures. Of these exposures, 82% occurred on four services: obstetrics-gynecology, surgery, medicine, and emergency medicine. The probability of exposure was not related to graduation year, clerkship location, previous clerkship experience, or training site. Surveys of two graduating classes at the beginning and end of the study showed that the percentage of exposures reported increased from 45% to 65% over the 7-year study period. Thus, the reported injury rates represent minimum estimates of actual occurrences. Human immunodeficiency virus infection and hepatitis were not reported, although follow-up was limited. Instruction in universal precautions and clinical procedures is not sufficient to prevent exposures to blood during medical training. Medical schools must assume greater responsibility for ensuring that students are proficient in the safe conduct of clinical procedures and must develop systems that protect students so that they can report and learn from their mistakes.