Social and medical consequences of 278 children and 264 adults injured in bicycle accidents and seen in two hospitals in Helsinki in 1985-86 were analyzed. Information was collected from patient records, by means of a special questionnaire and by telephone interview. A child outpatient required 1.7 and a child inpatient 3.0 physician visits on an average, while adults required 2.2 and 4.9 visits, respectively. The average duration of hospital stay was 8 days for hospitalized adults and 6 days for children. Rehabilitative care outside the hospital was received by 6% of the adult outpatients and 25% of the inpatients, but none of the injured children. The mean duration of work disability was 82 days among inpatients, 11 days among outpatients, 127 days among the inpatients injured in motor vehicle collisions and 65 days among inpatients injured in other bicycle accidents. Of inpatients 32% and of outpatients 5% reported persistent (> 6 months) disability. Persistent disability was recorded in 11% of children, in 47% of adults and in 67% of elderly inpatients. Most serious consequences were due to intracranial injuries in motor vehicle-bicycle collisions. Of the hospitalized bicyclists 4% suffered from severe cognitive and behavioural changes or sense impairment and of adult inpatients 3% suffered from permanent work disability. The average costs of health and social services were about FIM 1000 per adult outpatient and FIM 13000 per adult inpatient. In prevention high priority should be given to motor vehicle collisions, head injuries and injuries among the elderly bicyclists.