Comprehensive electron-microscopic studies showed that the structure of the collagen fibrils in the osteoid is not uniform. In 82 cases of osteogenesis imperfecta (OI), the diameter of the collagen fibrils was determined morphometrically under standardized magnification. The morphometric investigations of collagen fibrils of the osteoid in OI show clear differences in the four subtypes and in comparison with the control group.