The incidence of fractures of the distal radius and ulna in a defined adult population was studied over one year. Freezing temperatures were usually associated with a fourfold increase in fracture incidence, but there was no other seasonal trend. Incidence in men showed no relation to age. In women incidence rates increased sharply between the age groups 40-44 and 55-59 years and then continued to rise less steeply until ages 65-69 after which rates stabilized at a lower level. This pattern contrasts with the age- and sex-specific incidence of fractures of the proximal femur and of falls at post-retirement ages, and supports the hypothesis that one cause of fractures of the proximal femur in old age is failure of protective reflexes.