Follow-up studies of bone mineral content in the radius were done in 82 postmenopausal women 4 to 10 years after the first examination. These patients were subdivided into four groups depending on the type of menopause (artificial or natural) and estrogen administration (treated or untreated). Bone mineral mass and combined cortical thickness decreased significantly in both groups of untreated women. Both mineral loss per year for the untreated women was -9.1 mg/sq cm for castrates and -6.9 mg/sq cm for those with a natural menopause. In neither group was the rate of loss correlated with age. The change in bone mineral mass per year in the estrogen-treated subjects (mean +3.25 mg/sq cm) differed significantly from that of untreated subjects (mean -7.99 mg/sq cm). The findings suggest that postmenopausal osteoporosis could be prevented by estrogen treatment.