The effect of the anti-oestrogen tamoxifen on bone density in post-menopausal women with stage II breast cancer (n = 35) was compared with two untreated hypo-oestrogenaemic groups (premature ovarian failure, n = 20; prolactinoma, n = 19) and a group of women with prolactinomas and normal oestrogen levels (n = 12). Bone densities were normal in the euoestrogenaemic groups, but significantly reduced in both hypo-oestrogenaemic groups. Repeat measurements were available for 31 of the 35 tamoxifen-treated subjects after an interval of 33.9 +/- 14.7 months. The anti-oestrogen action of tamoxifen was not associated with accelerated bone loss of the spine or hip, an outcome that was clearly demonstrable in hypo-oestrogenaemic disorders.