Head injuries account for the great majority of bicycling deaths and hospital admissions. Helmet use has been suggested as an effective means of preventing injury, but few studies have addressed this issue. We collected questionnaire data by mail from 191 recreational bicyclists who reported having fallen and struck their heads in a cycling mishap. Information was collected on the nature of the mishap, the extent of injury, and the helmet status of the rider at the time of the fall. Fifty-seven percent of riders were wearing helmets during the mishap. Helmet wearers were significantly older than those not wearing helmets. Helmet wearers experienced significantly fewer skull fractures (1% versus 11%) and facial soft tissue injuries (5% versus 18%) than those not wearing helmets (chi 2 = 6.7, 6.5; P = 0.01, 0.01). No other variables accounted for differences in injuries. These data support the contention that bicycle helmets are effective in preventing head injuries.