Treatment of 63 oophorectomised women with oestrogen for five years prevented the reduction in bone mineral content observed in 57 women treated with a placebo preparation. When onset of therapy was delayed for three to six years there was a highly significant increase in bone mineral content mainly during the first three years of treatment. During the next two years there was no further increase in bone mineral, while the placebo-treated groups continued to lose bone at about 1% per annum. In association with the changes in bone mass, the expected biochemical effects of oestrogen therapy also persisted for at least three years, and were compatible with a prolonged increase in parathyroid activity but a reduction in bone turnover.