Normal skull thickness has been measured in a general hospital population of 300 blacks and 200 whites in America. In both groups, there is a rapid increase in skull thickness during the first two decades of life, followed by a small uniform increase reaching a peak in the fifth and sixth decades. The sex differences are variable, but in certain age groups the females in both races have significantly thicker parietal and occipital bones than their male counterpart. The frontal bone is thicker in the white male than in the black, and the parieto-occipital thicker in the blacks than in the whites. Some suggestions are offered to explain the sex and racial differences noted.