In patients with osteogenesis imperfecta (OI), a disease characterized by abnormal bone fragility, bone mineral density (BMD) was found to be relatively preserved. Quantitative computed tomography (QCT) is the only available method for directly measuring in vivo both volumetric density and the cross-sectional area. Here we report the data from dual-energy X-ray absorptiometry DXA (spine and hip) and peripheral (pQCT) (ultradistal and proximal radius) measurement of 27 adult patients affected by OI, mostly of type I, compared with a group of healthy persons. In the patients with OI, areal BMD values at both femoral neck and lumbar spine were considerably lower than in control subjects (-32 and -36%, respectively; p<0.001 for body weight and height adjusted values). pQCT volumetric density at the ultradistal radius was 19% lower than in control subjects and this difference rose to 32% for purely cancellous bone tissue. The whole bone cross-sectional area of ultradistal radius, as measured by pQCT, was superimposable to normal. At the proximal radius, both cross-sectional area and cortical area, together with Bending Breaking Resistance Index (BBRI), were significantly lower in OI (-23; -22; -32% respectively; p<0.001 for body weight and height adjusted values), but cortical volumetric density was even slightly higher in the OI group than in control subjects. In conclusion, it appears that the most obvious defect in adults with OI is the inability to acquire an adequate thickness of the cortices of long bone and to achieve or maintain normal trabecular density.