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Men with one extra X chromosome (47,XXY; Klinefelter syndrome) show a tendency to have a shallower but longer palate than normal (46,XY) men. The palate is also narrower than in 46,XY men. The mandible is clearly narrower but sagittally longer compared with the mandibles of 46,XY men. The present results indicate that the presence of one extra X chromosome in 47,XXY men is reflected in decreased growth of the maxilla transversely and vertically and of the mandible transversely. Increased length of the alveolar arches might be partly a compensation for the decreased width of the alveolar arch. This change might be associated with larger tooth sizes in 47,XXY men. Unbalanced appositional growth rather than disturbances in other orofacial growth mechanisms might be associated with the XXY sex chromosome pattern.