An iliac bone biopsy from an adult male, 58 years of age, with osteogenesis imperfecta type I was studied by bone histomorphometry after double-fluorescence labeling with tetracycline. Low bone mineral density (BMD) of the radius, measured by dual-energy X-ray absorptiometry (DXA) was associated with high levels of urinary deoxypyridinoline and serum bone-specific alkaline phosphatase and osteocalcin. At the tissue level, low cancellous bone volume (BV/TV) was associated with increased eroded surface (ES/BS) and a relatively increased osteoclast number (N.Oc/BS). Osteoid thickness (O.Th) was also decreased as a result of decreased bone matrix synthesis, in terms of decreased osteoblastic activity. However, osteoid surface (OS/BS) and osteoblast surface (ObS/BS), in terms of the number of osteoblasts, were increased. We conclude that the patient showed cancellous osteopenia, which was likely due to increased bone resorption with decreased activity and increased recruitment of osteoblasts.