Twelve infants, initially considered to have thanatophoric dysplasia, were studied by a combined radiographic-histochemical-biochemical approach. Three distinct forms of platyspondylic lethal neonatal short-limbed dwarfism could be distinguished: (1) Thanatophoric type, (2) Torrance type, and (3) San Diego type. The latter two disorders had similar radiographic abnormalities that were clearly different from those of typical thanatophoric dysplasia. All three disorders had clearly different condroosseous histopathologic abnormalities. Preliminary biochemical studies have revealed different electrophorectic abnormalities in solubilized type II collagen chains of cartilage in each of these three disorders.