TRANSSEXUALS have the strong feeling, often from childhood onwards, of having been born the wrong sex. The possible psycho-genie or biological aetiology of transsexuality has been the subject of debate for many years^1,2. Here we show that the volume of the central subdivision of the bed nucleus of the stria terminalis (BSTc), a brain area that is essential for sexual behaviour^3,4, is larger in men than in women. A female-sized BSTc was found in male-to-female transsexuals. The size of the BSTc was not influenced by sex hormones in adulthood and was independent of sexual orientation. Our study is the first to show a female brain structure in genetically male transsexuals and supports the hypothesis that gender identity develops as a result of an interaction between the developing brain and sex hormones^5,6.