Out of 21 male patients with osteoporosis who visited an outpatient clinic for endocrine diseases in two years (1994-1995), three had systemic mastocytosis as diagnosed histopathologically. Two of these had characteristic features of urticaria pigmentosa, consisting of multiple brown nodules on the skin of trunk and extremities, and a positive Darier sign. In all of them the excretion of the histamine metabolites methylhistamine and methylimidazoleacetic acid in a 24-hour urine specimen was increased. When osteoporosis is diagnosed in men or premenopausal women, underlying pathology could be considered. Cautious investigation of signs and symptoms of systemic mastocytosis in such patients might prove this disease be less rare than is often assumed.