We questioned members of 2 southern California off-road bicycling organizations about injuries associated with the use of all-terrain bicycles. Cyclists were asked about riding and safety habits, the kind(s) of injury sustained with their most recent accident and whether they sought medical treatment, and the circumstances of the accident. Of 459 mailed surveys, 268 (58.4%) were returned. Respondents (82.8% of whom were male) ranged in age from 14 to 68 years. Of these, 225 (84%) had been injured while riding all-terrain bicycles, 51% in the past year. Although most injuries were characterized as minor, 26% required professional medical care, and 4.4% of those injured were admitted to hospital. Extremity injuries--abrasions, lacerations, contusions--occurred in 201 (90%) cyclists with 27 (12%) sustaining a fracture or dislocation. High levels of helmet use (88%) may explain the low occurrence of head and neck trauma (12%). Frequent riding and riding on paved terrain were associated with increased severity of injury, although most accidents--197 (87.6%)--occurred off paved roads. These results suggest that, compared with regular bicyclists, all-terrain cyclists have more, but not necessarily more severe, injuries. Clinicians and emergency medical personnel should be aware that the increasing popularity of off-road cycling may change the frequency and nature of bicycling injuries.