The treatment of transsexual subjects is a challenging task for the endocrinologist who, in collaboration with the mental health professional and the surgeon, is called upon to confirm the diagnosis and adjust hormonal treatment aimed at suppressing endogenous sex hormones and to develop hormone characteristics of the desired gender. These guidelines are structured to provide evidence-based suggestions or, where evidence is lacking, expert recommendations on diagnostic procedures and hormonal treatment in adolescent and adult transsexuals, including long-term care and eligibility for surgery. The multidisciplinary approach to treatment, the additional diagnostic role of hormone administration and the need to maintain hormone levels within the physiological range are key suggestions stressed in the guidelines which are particularly important for an endocrinologist unfamiliar with this field. The need for psychological assessment before surgery is not common in many countries and should be stressed further in the guidelines. Some important issues such as time and method of hormone withdrawal before surgery together with when and which hormones should be administered after sex reassignment surgery has been completed also remain unclear. These guidelines represent a pivotal document for endocrinologists setting a standard for the care of transsexuals and providing directions for future research.