Estrogen deficiency has recently been implicated in the pathogenesis of male osteoporosis. We therefore investigated estrogen and androgen status in 63 men admitted to our clinic with the diagnosis of osteoporosis over a period of 2 years. The diagnosis was based on the presence of either low-energy fractures of the spine or a BMD T-score <-- 2.5 in the spine or hip. Thirty-six patients had one or more low-energy fractures of the spine, 47 displayed a lumbar BMD T-score <-- 2.5 and 39 a hip BMD T-score <-- 2.5. Based on the history, clinical examination and extensive biochemical testing, 42 of the 63 were classified as having primary osteoporosis. Of these 42 patients, 14 (33%) exhibited serum estradiol levels below the normal range (p<0.001). Two of the patients (3%) displayed male hypogonadism with serum testosterone below the normal range. In 37 of the 63 patients a complete estrogen status was available. In this group 26 were classified as having primary osteoporosis. Of these, no single case of male hypogonadism was demonstrable, while 10 (38%) exhibited undetectable serum estradiol levels (<48 pM). Thus, estrogen deficiency is much more prevalent than androgen deficiency in primary male osteoporosis. Future screening tests for osteoporosis in men should therefore include assessment of serum estradiol.