The paper presents the course ofschizophrenia in 27-year-old monozygotic femaletwins with similar symptomatology and response totreatment. In the treatment with risperidone in thedaily dose of 2-3 mg dyskinesia was noted. An ambivalent attitude towards this treatment has notbeen changed when the risperidone dose reached4 mg. In both patients hyperprolactinaemia over2500 mIU/L was found then, as well as galactorrheaand amenorrhea. The treatment with risperidonewas discontinued for this reason. Thetreatment and rehabilitation methods administeredin the years 1994-1999 are discussed. Routinemonitoring of prolactine levels prior to and duringthe treatment with risperidoneis recommended.