Hyperprolactinemia is a frequent consequence of treatment with typical antipsychotic agents and atypical antipsychotics such as risperidone. Recent studies have suggested that aripiprazole, a partial dopamine agonist, reduces the prolactin response to antipsychotics. Thus, we conducted this study to evaluate the dose effects of adjunctive treatment with aripiprazole on hyperprolactinemia in stable schizophrenic patients maintained with risperidone.