OBJECTIVE: Low bone mass may be caused by a reduction in the amount of bone or density of bone or both. The purpose of this study was to examine differences in bone volume and volumetric bone mineral density (vBMD) in men with primary osteoporosis and their first-degree male relatives (FDMR). DESIGN: We used dual-energy X-ray absorptiometry (DXA) to measure areal density, then calculated bone volume and volumetric density in 121 men with primary osteoporosis, 73 FDMR and 66 normal men. We used regression methods adjusting for age, height and weight to determine deficits in bone volume and vBMD at the spine and femoral neck between men with spinal fractures due to primary osteoporosis, FDMR and normal men. RESULTS: Men with osteoporosis had a tendency to smaller bone volume in the spine and femoral neck (P = 0.08 and P = 0.09, respectively) and lower volumetric bone density at the spine (by about 50%) and femoral neck (by about 30%) compared with healthy controls (P < 0.0001). FDMR had no deficit in bone volume but did have lower volumetric density at the spine (by 10.2%) compared with healthy controls (P < 0.0001). CONCLUSIONS: A deficit in bone mineral accrual may underlie the pathogenesis of primary osteoporosis in men, resulting in low vBMD. This is likely to be determined by genetic factors, although shared common environmental factors may also be important.