The pathogenesis of male osteoporosis at the cellular level is still elusive. We performed histomorphometric analysis of bone biopsy samples from 51 eugonadal men with idiopathic osteoporosis. Their median age was 54 (range 29-73) years. Eighty-two percent of the patients had a fracture history, and 57% had vertebral fractures. Bone volume, trabecular thickness, wall thickness, and osteoid thickness were significantly reduced in osteoporotic men compared with healthy men. Erosion depth was similar, as were the bone remodeling parameters such as bone formation rate, mineral apposition rate, and activation frequency. In the osteoporotic men, osteoid thickness was correlated to bone mineral density at the lumbar spine (R(2) = 0.19, P < 0.01); together with wall thickness, the two parameters could explain 27% of the variation in lumbar spine bone mineral density. The osteoid thickness was correlated to anthropometric variables such as body weight (R(2) = 0.24, P < 0.001) and body mass index (R(2) = 0.14, P < 0.01), as well as to serum estradiol levels (R(2) = 0.14, P < 0.01) and to the ratio insulin-like growth factor-1 (IGF-1) to IGF-binding protein-1 (IGFBP-1) (R(2) = 0.12, P < 0.01). Regression analysis showed that 36% of the variation in osteoid thickness could be predicted by body weight and estradiol levels. In conclusion, bone histomorphometry in male idiopathic osteoporosis was characterized by thin bone structural units, which might suggest osteoblast dysfunction. Bone histomorphometry parameters were associated with low body weight, low estradiol levels, and increased levels of IGFBP-1, supporting the notion that estrogens and IGFs play regulatory roles in male bone turnover.