A large family with Ehlers-Danlos syndrome type IV (EDS IV) has previously been described. Unlike most cases of EDS IV, fibroblasts from affected members secreted near normal amounts of type III collagen. We have localized the mutation in this family to the CB5 peptide of type III collagen, by using both protein and cDNA mapping techniques. Sequence analysis of cDNA revealed a 27-bp deletion within exon 37, a deletion that removed nine amino acids and maintained the Gly-X-Y repeat of the collagen helix. Further sequencing of genomic DNA confirmed its location, and amplification of DNA from family members showed that it was absent in unaffected individuals but present in all the affected individuals tested. This deletion is flanked by two short direct repeats of CTCC; it may have arisen by slipped mispairing, and has subsequently been transmitted to all affected family members.