The Ehlers-Danlos syndrome type IV represents a heterogeneous group of diseases, the molecular defect of which seems to reside in a defective synthesis and processing of collagen III. Here we present data concerning both protein and mRNA levels of collagens I and III in cell cultures established from skin of nine patients and six controls. All patients investigated were characterized by a reduced amount of procollagen III in fibroblast cultures. However, the levels of collagen III propeptides, measured by a radioimmunoassay in culture medium of fibroblasts from the patients, were either decreased or in the range of control levels. Using hybridization with cDNA probes specific for pro alpha 1 (I) and pro alpha 1 (III) collagen chains we determined the ratios of procollagens I and III mRNA. Although synthesis of procollagen III was markedly reduced in fibroblasts of all patients studied, no parallel decrease of procollagen III specific mRNA was found. However, two distinctive groups of patients were identified, one with an unaltered ratio of procollagen I/III mRNA and the other which had an even higher proportion of collagen III mRNA.