Three clinicopathological observations of a mild form of type II achondrogenesis are presented. The cases were selected from a group of 21 similar cases to illustrate the various degrees of clinical and roentgenological signs that can be found. The cases had various survival periods after birth but not exceeding several months. The roentgenological signs were less severe than those of type II achondrogenesis. Some cases similar to case no. 3 have roentgenological signs very close to spondylo-epiphyseal dysplasia congenita and probably were confused previously with the latter. The name of hypochondrogenesis was proposed for these cases because the lesions of the growth plate are similar although less marked to those found in type II achondrogenesis: high cellularity with poor matrix development; irregular columnization and vascular penetration; large chondrocytes and even more enlarged lacunae; large sclerotic cartilage canals. The clinical and roentgenological diagnosis of hypochondrogenesis could be difficult especially in the less severe forms. The delay in vertebral ossification, the absence of all the epiphyseal nuclei and of the tarsal bones might suggest the diagnosis of hypochondrogenesis, rather than that of spondyloepiphyseal dysplasia. The evolution which seems to be always lethal in a period of several weeks or months would make the diagnosis still more likely and it could be confirmed by histopathological examination. Cases of spondylo-epiphyseal dysplasia congenita might have at birth, roentgenological signs indistinguishable from those of hypochondrogenesis, as was illustrated by case no. 4.(ABSTRACT TRUNCATED AT 250 WORDS)