Sex differences in second (2D) and fourth (4D) fingertip extensions relative to the middle fingertip and 4D:2D fingertip extension ratios were Studied in men and women. Body height positively correlated with index fingertip extensions, not with ring fingertip extensions, nor with their ratio. Mean 2D extension (both hands) was smaller in women than men; mean 4D extension (right hand) was smaller in men than women; 4D:2D fingertip extension ratios from both hands were larger In women than men. Serum estradiol concentration negatively correlated with 2D extensions for both hands (no significant correlation with 4D extensions), but positively correlated with 4D:2D extension ratios for both hands. Serum testosterone concentration positively correlated with 2D extensions of both hands (no significant correlation with 4D extension), but negatively correlated with 4D:2D extension ratios for both hands. These relations were also studied in men and women separately. It was Concluded that the 4D:2D extension ratio was greater in women than in men: 2D and 4D extensions and 4D:2D extension ratios may be determined prenatally by sex hormones; fingertip extensions may be predictive of adult and prenatal sex hormone levels.