Sex chromatin counts for more than 50 individuals with three or more X chromosomes in their complement were analyzed. The observed distribution of cells with 0, 1, 2,...n sex chromatin bodies in polysomic-X individuals was compared with the binomial distribution expected on the assumption of a constant probability that an X chromosome will form a visible sex chromatin body. The sex chromatin count distribution deviated significantly from a binomial distribution in approximately half of the cases, usually in the direction of an excess of cells in the extreme classes (with 0 or n sex chromatin bodies, where n is usually 2 or 3). This indicates that the probability that an inactive X chromosome will form a visible sex chromatin body is not constant but varies, perhaps in relation to the cell cycle. The existence of cells with less than the maximum number of sex chromatin bodies therefore cannot be due solely to technical factors acting at random on all inactive X chromosomes. The sex chromatin count distribution appears to be different in newborn and postmenopausal XXX females, especially the latter, where nearly half had a deficiency of cells in the extreme classes. Most of this subgroup had sexual abnormalities, and many were schizophrenic. The types of deviation from the binomial distribution which would be produced by between-cell and within-cell variation in the probability of sex chromatin body formation, by mosaicism and by activation of an X chromosome are outlined. Various biological interpretations of the observed deviations are discussed, including the role of the cell cycle, hormones and other environmental factors.