The association between taurodontism and extra X chromosomes was studied in four 47,XXX-females and in two 48,XXXX-females. Occurrence of the trait in the permanent mandibular molars was noted from orthopantomograms. Five first-degree relatives and a sample of 157 normal males and females were investigated as controls. Two of the 47,XXX-females and both 48,XXX-females each had at least one mandibular molar classified as taurodont. The two affected 47,XXX-females had hypotaurodont or mesotaurodont teeth, whereas both 48,XXXX-females showed hypertaurodontism. The manidubular molars of the other two 47,XXX females had normal root morphology. The only control relative with taurodont teeth was a sister to a 48,XXXX-female. In the population control group, four females had taurodont teeth. These results support the concept that a prevalence of taurodontism increases as the number of X chromosomes increases and also indicate that expression of the trait and the number of X chromosomes may be positively correlated. It is suggested that the X chromosome gene(s) influencing development of enamel may be involved in the development of taurodontism.