BACKGROUND The aim of the study was to analyze the morphology of dental arches and skeletal mandibular-maxillary bases in untreated pseudo-Class III and true Class III malocclusions, by using posteroanterior radiographs and 3-dimensional digital models. METHODS The records of 50 untreated patients (24 boys and 26 girls between 14 and 16 years of age) with Class III malocclusions were included in this study. They were divided into 2 groups according to their Class III malocclusion type: true or pseudo-Class III malocclusion. Maxillary skeletal base, and bigonial and biantegonial widths were measured on posteroanterior cephalograms. Also, maxillary and mandibular intermolar widths were calculated on 3-dimensional digital models. For the statistical evaluation, the independent-samples t test was used, and the Pearson correlation coefficient was calculated to determine the relationship between the dental and skeletal widths. RESULTS Maxillary intermolar, and bigonial and biantegonial widths in true Class III malocclusions were significantly larger than those in pseudo-Class III malocclusions (P <0.05). On the other hand, maxillary skeletal base width in pseudo-Class III malocclusions was significantly larger than in true Class III malocclusions (P <0.05). Also, mandibular intermolar widths in true Class III malocclusions were larger than in pseudo-Class III malocclusions, but the difference was not significant (P >0.05). Significant positive correlations were determined not only between the maxillary and mandibular molar widths but also between the bigonial and biantegonial widths (P <0.01). CONCLUSIONS The morphologies of the dental arches and the skeletal mandibular-maxillary bases differ in a significant manner between untreated pseudo-Class III and true Class III malocclusions.