Recently, a decrease in bone mineral content (BMC) in hyperprolactinaemic women with long-lasting amenorrhoea has been reported, and attributed either to a direct effect of PRL on bone or secondary to the oestrogen deficiency. To verify if PRL by itself has a direct effect on bone, we have studied BMC at the lumbar level by double-photon absorptiometry in 22 patients with hyperprolactinaemia, selected on the basis of normal or near-normal oestradiol levels. The results were compared with those obtained in 28 healthy closely-matched women, and seven hyperprolactinaemic patients with long-lasting amenorrhoea and oestrogen deficiency. No significant difference in BMC was observed between hyperprolactinaemic patients with normal oestrogen levels (mean +/- SEM = 3.87 +/- 0.10 gHA/cm) and normal subjects (mean +/- SEM = 3.76 +/- 0.10 gHA/cm). Moreover, no significant change was observed during a 6 month follow-up in 13 patients. On the other hand, a significant difference (P less than 0.05) was detected in BMC between the hyperprolactinaemic patients with normal oestradiol levels and those with long-lasting amenorrhoea and oestrogen deficiency (mean +/- SEM = 3.39 +/- 0.18). These results suggest that hyperprolactinaemia by itself is not a risk factor for the development of osteoporosis.