The immunohistochemical localization of Type III collagen was studied in the dentin from nine patients with dominantly inherited osteogenesis imperfecta (O.I.) and six patients with hereditary opalescent dentin (H.O.D.). The most consistent localization of Type III collagen was found in patients with O.I. Among these patients an indirect relationship existed between the severity of dentinal involvement and the presence of Type III antibody localization. Although a similar but less intense staining could be observed in some cases of H.O.D., the severity of dentinal alterations in H.O.D. was not correlative to the quantitative presence of material reacting with antibodies to Type III collagen. These observations further indicate that O.I. is not only clinically, but biochemically, a heterogeneous condition.