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Permanent teeth of 12 individuals with a 47,XYY chromosome constitution have been examined. The tooth sizes of 47,XYY males were found to be larger than those of control males and females. In many instances the differences were statistically significant. Using these results, it was possible to conclude that a factor or factors which influence excess growth of 47,XYY males probably are in effect during prenatal life, but without doubt must be in effect very early in postnatal life. The time period needed for the achievement of final excess growth is relatively short, in the case of first permanent molars probably only from 2 1/2 to 3 1/2 years. On the basis of the finding that the Y chromosome apparently carries genes affecting tooth sizes in normal males [1], it was suggested that gene products of the extra Y chromosome could cause the observed size difference between normal and 47,XYY males. The nature of the influence of one versus two Y chromosomes on growth was discussed in terms of the possible influence of the Y chromosome on the cell divisions within the developing tooth germ.