The literature concerning endochondral histopathology in the chondrodystrophies is misleading because the true heterogeneity of this group of disorders was not recognized until relatively recently. Accordingly, biopsies of the chondro-osseous junctions of rib and iliac crest were performed on seven typical achondroplastic dwarfs ranging in age from 14 months to 35 years. In contrast to previous reports, histologic studies of this biopsy material demonstrate regular, well organized endochondral ossification in classic achondroplasia. These observations suggest that the basic defect in achondroplasia may be a quantitative decrease in the rate of endochondral ossification, which, in conjunction with undisturbed periosteal bone formation, results in the short, squat shape of the tubular bones. These findings necessitate a reclassification of the many animal chondrodystrophic mutants, some of which have been mislabeled as models of achondroplasia because of their abnormal endochondral histopathology.