The scientific community is very interested in the biological aspects of gender disorders and sexual orientation. There are different levels to define an individual's sex: chromosomal, gonadic, and phenotypic sex. Concerning the psychological sex, men and women are different by virtue of their own gender identity, which means they recognize themselves as belonging to a determinate sex. They are different also as a result of their own role identity, a set of behaviors, tendencies, and cognitive and emotional attitudes, commonly defined as "male" and "female". Transsexuality is a disorder characterized by the development of a gender identity opposed to phenotypic sex, whereas homosexuality is not a disturbance of gender identity but only of sexual attraction, expressing sexual orientation towards people of the same sex. We started from a critical review of literature on genetic and hormonal mechanisms involved in sexual differentiation. We re-examined the neuro-anatomic and functional differences between men and women, with special reference to their role in psychosexual differentiation and to their possible implication in the genesis of homosexuality and identity gender disorders. Homosexuality and transsexuality are conditions without a well defined etiology. Although the influence of educational and environmental factors in humans is undeniable, it seems that organic neurohormonal prenatal and postnatal factors might contribute in a determinant way in the development of these two conditions. This "organicistic neurohormal theory" might find support in the study of particular situations in which the human fetus is exposed to an abnormal hormonal environment in utero.