To obtain epidemiologic data and information on the probable causes and severity of bicycle-related injuries, we interviewed all patients with such trauma. Between April 1 and Oct 1, 1983, 520 children presented to the Emergency Department of The Children's Hospital of Philadelphia with trauma related to two-wheeled nonmotorized bicycles. The ages of the patients ranged from 1 to 18 years (mean, 8.7 years), and most (72%) were males. Most accidents (84%) occurred less than five blocks from home, and 49% occurred in the street. Thirty-six percent of the patients admitted to stunt riding or going too fast when the accident occurred, and 36% claimed there was a problem with the surface on which they were riding. The accidents occurred when a patient lost control of the bike (45%), a patient on a bicycle was hit by a car (17%), or a pedestrian was hit by a bicyclist (10%). Only three patients were wearing protective equipment at the time of the accident. Most (54%) had received no specific safety instructions about bicycling. The extremities were injured in 53% of the accidents, but head and neck injuries accounted for 31%. Six percent required hospital admission. Males and children over 12 years of age were more likely to have multiple injuries. Accidents that occurred in the street or involved cars were associated with a greater number of serious and multiple injuries. The infrequent use of protective equipment and minimal safety instructions received by the patients in this study suggest that many bicycle-related injuries are preventable. Education of parents and children is recommended to improve bicycle safety.