There is evidence that the brains of men and women are structurally different, but there are few data regarding possible sex differences in white matter microstructure. Using diffusion tensor imaging we assessed fractional anisotropy (FA) in the frontal lobe white matter on contiguous 5 mm slices in nine healthy male and 11 healthy female adults. Overall, women had higher FA in the left frontal lobe compared to men and a leftward asymmetry of FA in contrast to men, who showed no hemispheric asymmetry. Among women, greater leftward asymmetry of frontal lobe FA correlated significantly with better verbal comprehension and memory functioning. Our findings may be indicative of increased directional coherence and/or density of left hemisphere white matter fibers and a leftward asymmetry of this structural integrity among women compared to men.