RESUMEN
The prevalence rate of transsexualism varies from 1 to 50,000, to 1 to 100,000. Although it remains an infrequent affliction, transsexualism generates usually major suffering and may be responsible of many complications like suicide, self-mutilations, affective disorders and social disabilities. Since the first descriptions of Esquirol in the nineteenth, the medical community has always been questioned on medical, legal, social or ethical aspects of transsexualism. The aetiology of the trouble is still unknown. In the absence of biological marker, the syndrome of transsexualism can be defined only with clinical criteria. The main differential diagnosis are sexual ambiguities and psychotic disorders. For the specialists, satisfying the patients demand of a surgical and social reassignment still remains the only way to improve their clinical condition and avoid the onset of many dramatic complications. Without any treatment, the evolution of the trouble is chronic, without remission. Longitudinal studies of transsexual patients with a five year follow-up demonstrated subjective improvement in two thirds of the patients and don't find either higher rates of suicides nor psychotic decompensations after surgery and hormonotherapy. Clinical and neuropsychological studies of sexually differentiated cognitive abilities of transsexual patients, before and after hormonotherapy, could allow us in improving the understanding of sexual differences of the brain.