Enamel thickness of the maxillary permanent central incisors and canines in 14 47,XYY males, their male and female relatives and population-control males and females were determined from radiographs. The results indicated, although not fully unambiguously, that the thickness of enamel and that of 'dentin' (distance between mesial and distal dentino-enamel junctions) are increased in the teeth of 47,XYY males compared with normal controls. Earlier results have indicated a direct growth-promoting effect of the Y chromosome on tooth growth by influencing both enamel formation and, possibly through cell proliferations, growth of dentin. The present results can be considered additional evidence for the presence of the factors within the Y chromosome controlling different growth processes. Tooth size measurements in two males with deletions of the parts of the y chromosome suggested that there may be a specific growth-promoting gene(s) in the non-fluorescent part of the long arm. It is suggested that the way of influence of the Y chromosome on the amelogenesis is regulatory, and that the difference in tooth size between males and females is explained by a differential growth-promoting effect of the Y chromosome compared to the X chromosome.