We reviewed clinical and biochemical findings from 132 probands with nonlethal forms of osteogenesis imperfecta (OI) whose fibroblasts were sent to the University of Washington for diagnostic studies in the years 1981-87. In cells from 86% of probands with nonlethal OI we identified biochemical alterations compatible with heterozygosity for a mutation that affected expression or structure of alpha chains of type I procollagen. We observed two major biochemical phenotypes. Cells from 40 probands (group A) secreted about half the normal amount of normal type I procollagen and no identifiable abnormal molecules; these patients were generally of normal stature, rarely had bone deformity or dentinogenesis imperfecta, and had blue sclerae. Cells from 74 probands (group B) produced and secreted normal and abnormal type I procollagen molecules; these patients were usually short and had bone deformity and dentinogenesis imperfecta, and many had grey or blue-grey sclerae. In cells from an additional 18 probands (group C) we were unable to identify altered type I procollagen synthesis or structure. Detection of these abnormalities has value in the determination of mode of inheritance and in the prediction of clinical severity.