Osteogenesis imperfecta is a clinically and genetically heterogeneous group of inherited connective tissue disorders in which bone fragility is the predominant feature. Cultured dermal fibroblasts from one patient with the lethal perinatal form of osteogenesis imperfecta secrete type I procollagen at a rate half that of normal cells. Short-term labeling experiments and treatment with alpha,alpha'-dipyridyl (which prevents posttranslational prolyl and lysyl hydroxylation) demonstrated that these cells produce two distinct pro alpha 1(I) chains, which are synthesized at the same rate. Analysis of cyanogen bromide peptides indicated that the two chains differ in their primary structures. Thus, structural abnormalities in type I procollagen prevent this molecule from being secreted normally, resulting in an anomalously low ratio of type I procollagen to other extracellular matrix molecules. While the lethal perinatal form of osteogenesis imperfecta may be heterogeneous, we propose that the underlying pathogenesis of at least one form is decreased secretion of type I procollagen.