Gender identity disorder (GID, transsexualism) is a multidisciplinary problem of an unclear aetiology. Although the diagnosis of GID is generally established by psychiatrists, the diagnostic team always includes an endocrinologist, who is responsible for hormonal therapy. Hormonal therapy is the first step in the sex reassignment procedure and requires careful monitoring in the initial phase and in later years of treatment. In this paper we review the latest aspects of the diagnosis and treatment of transsexualism and the most common complications of hormonal intervention.