Eighty-eight medical students, interns, and residents were surveyed to study the epidemiology of their percutaneous exposures to blood. Respondents described 159 injuries in 221 person-years (py) of exposure in hospital wards and 213 injuries in 166 py of exposure in operating rooms. Nearly all injuries (greater than 98%) were needlesticks; less than 5% were reported to occupational health services. Rates of ward-related injury were highest for students (0.97/py) and decreased during training. Most injuries were due to recapping of used needles. In contrast to ward-related injury, rates of operating room-related injury were relatively low for nonsurgical students and interns (0.3/py), higher for surgical students (1.36/py), and stable over surgical residency training (mean, 5.4/py). Virtually all surgical injuries occurred during suturing. Further research into mechanisms of needlestick injuries and product design for their prevention are needed.