Sixteen patients, eight severely affected (osteogenesis imperfecta congenita), seven moderately (osteogenesis imperfecta tarda), and one mildly (no bone deformity) had iliac-crest biopsy specimens analyzed quantitatively to determine number of osteocytes, fractional areas of bone, length of resorption surface, and other histological parameters per area of the histological preparations. The osteocytes were profuse and the fractional areas of bone were diminished in all cases as compared with controls, and the findings correlated with the clinical severity of the disease. Patches of fiber bone were the only marked abnormality found in the mild case, although the parameters mentioned differed from normal controls significantly.