Classical antipsychotic drugs increase the level of serum prolactin. The atypical antipsychotic clozapine barely increases prolactin levels. An open naturalistic study in the University Hospital of Groningen suggests that treatment with risperidone in comparison to classical antipsychotics seems to increase the risk of erectile and ejaculatory disorders in male patients and to menstrual disturbances in female patients. In the literature reviewed, risperidone is associated with higher prolactin levels in comparison to classical antipsychotics. It is still unclear which mechanism is responsible for the increase of serum prolactin levels and sexual dysfunctions in patients using risperidone. Dopaminergic and serotonergic receptor blockade seems to play an important role. Other mechanisms are discussed.