Achondrogenesis has traditionally been divided into type I (Parenti-Fraccaro) and type II (Langer-Saldino). We studied the clinical, radiologic, and morphologic features of 17 cases previously diagnosed as achondrogenesis type I to define whether there is even further heterogeneity. On radiographic analysis, two distinct groups of patients were defined based on the presence or absence of rib fractures and ossification of the vertebral pedicles, ischium, and fibula. Two distinct chondroosseous morphologic patterns were observed that directly correlated with the radiographic grouping. One group had round vacuolated chondrocytes with inclusion bodies; the other had collagenous rings around the chondrocytes. We conclude that achondrogenesis type I (Parenti-Fraccaro) consists of two distinct disorders: type IA, which corresponds to the cases originally published by Houston et al. and Harris et al., and type IB, which corresponds to the case originally published by Fraccaro. Analysis of Parenti's case suggests the diagnosis of achondrogenesis type II. All three types of achondrogenesis appear to be inherited as autosomal recessive traits.