The prevalence of Klinefelter's Syndrome in new-born males in large surveys throughout the world has shown a scatter of 1.4 to 1.9 per thousand. Forssman (1970) accepted by pooling results a figure of 0.17 per cent as the average proportion of males born with at least one too many X chromosomes in some or all cell lines. Studies of mental hospital populations have consistently shown a higher prevalence of extra X chromosomes in males, averaging 0.54 per cent. The two largest studies, in Great Britain (Maclean et al, 1968) and in Sweden (Hambert, 1966) made no differentiation in the psychiatric diagnosis of the patients, although Hambert stated that the high prevalence did not stem from the mentally retarded population.