We studied the epidemiology of ankle fractures prospectively during 1 year in a population of about 200,000. The overall incidence rate was 107 fractures per 10(5) person-years. Below the age of 50, ankle fractures were commonest in men. After this age, females became predominant and the age-specific incidence rates decreased in both sexes. The main cause of fracture was falls (87%), on the ground, on stairs or from a height. 137 fractures (55%) occurred in sports, play or other leisure activities. Most patients (64%) were walking, running or jumping at the time of injury. Alcohol and slippery surfaces were each involved in nearly a third of the cases. The distribution of the fractures according to both the Lauge-Hansen and the AO Weber classification systems were within the limits of previous series. Nearly half the patients were hospitalized and the fractures were operated on with osteosynthesis. Our findings indicate that ankle fractures are mainly caused by substantial trauma sustained during physical activity. Osteoporosis seems to be of minor importance.