The aim of the present study is to determine the effect of the atypical antipsychotic drug, risperidone on sleep measures in patients with schizophrenia by polysomnography. Sleep measures were compared in five schizophrenic patients who were receiving risperidone alone and five schizophrenic patients who were receiving haloperidol alone. There were no differences between these two groups in their demographic characteristics or doses of antipsychotic medication. The slow wave sleep period was significantly longer in the risperidone-treated group than in the haloperidol-treated group. There were, however, no other significant differences in sleep variables between these groups. This difference in the effect on sleep between risperidone and haloperidol may be due to their differential actions on serotonin (5-HT2) receptors. Risperidone, which is known to be a serotonin-dopamine antagonist, has the potential to improve the quality of sleep in schizophrenic patients.