We performed a population-based study of bicycle-related brain injuries in San Diego, California, residents during 1981. Incidence rates among males were three times higher than for females and were highest at ages 10-14 years for males. Only one-third of bicycle-related brain injuries involved collision with a motor vehicle, and this proportion was independent of age or gender. Brain injuries from motor-vehicle collisions were more severe than those resulting from other causes. Over half the brain-injured bicyclists aged 15 and older who were blood alcohol tested were legally intoxicated.