In young hyperprolactinaemic patients a reduction of bone mass density is frequently shown. However, until now the real mechanism has not been clarified (a direct role of PRL has been suggested). To better clarify the origin of the bone demineralization during hyperprolactinaemia we evaluated the BMD in a group of 24 proved PRL secreting pituitary adenomas. Based on menstrual characteristics the patients were subdivided in 3 groups: 1) oligomenorrhoea (OM), 2) amenorrhea lasting less than 2 years (AMa), 3) long-lasting amenorrhea (AMb). Twelve women with normal menstrual cycles served as controls. The BMD values at L2-L4 and thighbone levels were significantly reduced in the AMb group with respect to the other subgroups. The results support the hypothesis that BMD reduction in aPRL patients is secondary to hypoestrogenism and to the duration of amenorrhea rather than to hyperprolactinaemia.