We determined the trauma type of femoral neck and trochanteric fractures in men and women aged greater than or equal to 20 years in 19, 128 fractures reported to a computerized medical information register during 1972-1981. We found that the earlier known exponential increase of fracture incidence by age started in early adulthood rather than during middle age. An age-related increase was found for each type of injury as well, although the incidence of fractures resulting from severe trauma increased less than that of fractures associated with moderate trauma. The incidence of low-energy fractures in men as well as in women displayed a constant increase rate with age from young adulthood on, impugning the widely held belief that the menopause is a significant risk factor for fractures. The dominating risk factor for proximal femoral fracture in adults appears to be age, irrespective of the cause of injury, with a chronological decline in bone strength as the most likely cause of the exponential increase in the fracture incidence during adult life.