Background and Objective - Amenorrhoea in women of reproductive age causes loss of bone mineral. This study assessed the effect of treatment of amenorrhoea on bone mineral density. Design - Serial measurements of bone mineral density were obtained in women receiving treatment for amenorrhoea. Patients - Eighty-five women aged 17-40 with a past or current history of amenorrhoea, from various causes, with median duration of 46.5 months (range 8 months-21 years). Measurements - Bone mineral density in the lumbar spine was measured by dual-energy X-ray absorptiometry. Results - Initial vertebral bone mineral density was low, mean 0.85 (SD 0.10) g/cm2. After an interval of 19.6 (SD 7.5) months on treatment there was a highly significant increase to 0.89 (SD 0.10) g/cm2 (P < 0.0005). This was equivalent to a gain in bone mass of 2.1% per year (95% confidence interval 1.5-2.8%). Improvement was seen in all diagnostic groups (except polycystic ovary syndrome) and with all types of therapy. We observed no difference in the response of previously untreated patients compared with those already on treatment, nor any change in response with increasing duration of treatment. No new fractures were reported during the study. Conclusions - Bone mineral density in young women with amenorrhoea is improved by appropriate treatment, but recovery is not substantial. Hence early diagnosis and therapy is essential to prevent bone loss.