Nineteen cases of chondrodysplastic short-limbed dwarfism in early infancy were studied in the context of current clinical classification based on established radiographic criteria. The histopathologic findings were sufficiently distinctive in most categories to provide additional diagnostic criteria and to contribute to the understanding and delineation of these disorders. 1. Homozygous achondroplasia is distinguished by markedly disturbed endochondral ossification that differs from the pattern seen in typical heterozygous achondroplasia. The physeal chondrocytes contain abundant granules of glycogen. 2. Achondrogenesis, the severest form of chondrodysplasia, exists in 2 types, in which the histopathologic findings appear to be distinctive and diagnostic. Type 1 is characterized by severe disturbance in endochondral ossification. The zone of resting cartilage contains relatively normal matrix, and the chondrocytes contain intracytoplasmic inclusions. The cartilaginous matrix in type 2 is markedly deficient, and the chondrocytes have a large, primitive, mesenchymatous appearance. 3. Thanatophoric dwarfism is associated with disorganized endochondral ossification similar to that in homozygous achondroplasia, but there is no accumulation of glycogen within chondrocytes. Type 2 thanatophoric dwarfism is differentiated from classical type 1 by the presence of cloverleaf skull and histologically by many bone-lined, penetrating vascular canals in the physis and by hyperactive osteoblasts and osteoclasts in the metaphysis. 4. Asphyxiating thoracic dysplasia of Jeune is differentiated histologically into 2 types. Type 1 is characterized by patchy distribution of endochondral ossification in the physis, irregular physeal-metaphyseal junction and large islands of poorly mineralized cartilage in the metaphysis. Type 2 is characterized by uniform distribution of endochondral ossification that is disorganized and is accompanied by advancing cartilage forming latticelike meshwork in the metaphysis. 5. Chondroectodermal dysplasia, which is radiographically similar to asphyxiating thoracic dysplasia, is marked by the presence of large islands of poorly mineralized cartilage in the spongiosa of vertebral bodies. Disorganized endochondral ossification is, however, uniformly distributed, and there is no latticelike advancing cartilage in the metaphysis. 6. Chondrodysplasia punctata is characterized by myxoid and cystic degeneration of physeal and epiphyseal cartilage with focal calcification. Type 2 (Conradi-Hünermann) has a nearly normal pattern of endochondral ossification, even though there may be severe disturbance and retardation of the process. The columnization of physeal chondrocytes is normal. Type 1 (rhizomelic) differs by having markedly retarded and disorganized endochondral ossification. 7...