Osteoporosis is associated with increased bone resorption together with a decrease in bone formation. In women, there is an increase of resorption surfaces and in the number of osteoclasts. These changes, however, are not found in males. This purpose of this study was to examine clinical, laboratory, and histomorphometric data in a series of 22 male patients with primary osteoporosis and normocalciuria undergoing transiliac bone biopsy. All of them gave written informed consent for bone biopsy. Automated biochemical profile, urinary calcium excretion, and bone marker assays were performed. Histomorphometric studies were carried out in transiliac bone biopsies obtained with a Bordier-Lepine needle with previous tetracycline labeling. The histomorphometric values of cancellous bone showed significantly lower values of bone volume and values of osteoid surface (OS/BS) and osteoblast surface (Ob.S/BS), and a modest increase in osteoid thickness (O.Th) without changes in the mineralization lag time or eroded surface in patients compared with controls. In cortical bone, there was a low cortical volume (Ct.V/TV) and cortical width (Ct.Wi) in patients compared with controls, without differences in cortical porosity (Po.V/TV). These results suggest that normocalciuric idiopathic osteoporosis in men is characterized by decreased cancellous osteoblasts and bone turnover.