The relation between sexual orientation and penile dimensions in a large sample of men was studied. Subjects were 5122 men interviewed by the Kinsey Institute for Research in Sex, Gender, and Reproduction from 1938 to 1963. They were dichotomously classified as either homosexual (n = 935) or heterosexual (n = 4187). Penile dimensions were assessed using five measures of penile length and circumference from Kinsey's original protocol. On all five measures, homosexual men reported larger penises than did heterosexual men. Explanations for these differences are discussed, including the possibility that these findings provide additional evidence that variations in prenatal hormonal levels (or other biological mechanisms affecting reproductive structures) affect sexual orientation development.