In men the risk of fragility fractures increases as bone mineral density (BMD) declines but the mechanical strength of vertebrae is also dependent on the trabecular architecture. We have examined the relationships between architectural changes of trabecular bone and vertebral crush fractures in 44 male patients with osteoporosis (bone mineral density more than 2.5 SD below the young adult value). Spine radiographs, hip and lumbar spine BMD and transiliac bone biopsies were obtained for all subjects. Histomorphometric study was done on an image analyzer and the following parameters were determined: trabecular bone volume (BV/TV), trabecular thickness (Tb.Th) and number (Tb.N), Trabecular Bone Pattern factor (TBPf), Inter Connectivity Index (ICI), characterization of the trabecular network (Node count and Strut analysis) and Star volume of the marrow space. Eleven male patients, who were referred to our unit for the diagnosis a monoclonal gammopathy of undetermined significance, were selected as controls. The mean values of Tb.Th, Tb.N and Node count were found lower while TBPf, ICI and Star Volume were significantly higher in men with osteoporosis. Exponential regressions were found to best describe the relationships between BV/TV and the architectural parameters: TBPf (r = 0.94 p = 0.01), ICI (r = 0.63 p = 0.001), Star volume (r = 0.79 p = 0.001). There were no significant differences in age, vertebral and hip BMD, BV/TV and Tb.Th between patients with or without fracture. In contrast, in patients with at least one vertebral fracture, ICI and TBPf were significantly higher while Nodecount was lower. Our data suggest that an altered trabecular bone architecture is a major determinant of osteoporotic fractures in men.