BACKGROUND Evaluations of the dental arch widths and mandibular-maxillary base are needed for a comprehensive dentofacial analysis in subjects with Class III malocclusion. The aim of this study was to analyze the development of the dental arches and the skeletal mandibular-maxillary bases in untreated subjects with Class III malocclusions. METHODS Two groups of subjects, 1 with Class III malocclusion and the other with Class I malocclusion, were examined. Maxillary skeletal base width, biantegonial widths, and maxillary and mandibular intermolar widths were determined on posteroanterior cephalograms at annual intervals between the ages of 10 and 14 years. RESULTS Maxillary skeletal base widths and intermolar widths in the Class III subjects were significantly smaller than those in the Class I subjects (P <.05). No statistically significant differences were found among the groups for skeletal mandibular width or intermolar width for the total observation period. The deviations in molar differences increased from ages 10 to 14 in the Class III group. CONCLUSIONS The main transverse deficiencies in the Class III group were maxillary deficiencies in both skeletal and dental widths. The deviations in molar differences appear to become larger from age 10 to age 14.