The objectives of this study were to describe bicycle-related injuries in relation to injury patterns, age, gender and medical treatment in a defined Swedish population and to identify factors contributing to injury. The study group comprised all patients living in the county of Västmanland, Sweden, visiting a physician or dentist because of bicycle-related injury during one year (November 1989-October 1990). Cyclists were mostly injured on pavements, pedestrian malls and cycle tracks. Twenty percent of the events occurred on public roads in urban areas; most frequently, the injured were in the age range 0-24. The most common bicycle injury event involved no other party. The events were often caused by environmental factors, in combination with behaviour such as excessive speed, lack of attention, breach of traffic regulations or a co-ordination problem. Head injuries, including oral injuries, were the most common, in particular among children and adolescents. One in four children in the age range 0-9 sustained an oral injury.