The relationship between serum testosterone level and nonverbal intelligence was studied in right-handed young adults with regard to handedness. Hand preference was assessed by the Edinburgh Handedness Inventory. Hand skill was measured by a peg moving task. Serum testosterone level was determined using tritium-marked-radioimmunoassay. Visual-spatial performance (nonverbal intelligence) was measured by Cattell's Culture Fair Intelligence Test. In men with consistent right-hand preference (GSs: 80 to 100), IQ was found to be positively linearly related to serum testosterone, which exhibited two regression lines belonging to low and high difference in skill between hands. In females with consistent right-handedness, there was a negative linear correlation between IQ and serum testosterone, which also exhibited two different regression lines according to difference in skill between hands. In males with moderate right-hand preference (GSs 50 to 75), IQ was found to be positively linearly related to serum testosterone, exhibiting two different (same slopes) regression lines according to difference in skill between hands. In females with moderate right-hand preference, IQ first increased and then decreased with serum testosterone, exhibiting a quadratic relationship. These results suggested that serum testosterone in young adults may be associated with visual-spatial performance depending upon sex, hand preference, and hand skill.