This study was designed to evaluate the relationship between nonverbal IQ and brain size in men and women. A significant correlation was found between IQ and brain size in total subjects. There was no significant correlation between these variables in women; men exhibited only a weak relation of brain size to IQ. In both left-handed men and women significant correlations were found between IQ and brain size. The results supported the hypothesis about the relation of IQ to brain size, suggesting that the sex difference in the cranial capacity and cerebral asymmetry might contribute to the diversity in cognitive functions.