Psychosis, linked to chronic levodopa and other antiparkinsonian drug treatments, is a common and incapacitating complication of advanced Parkinson's disease (PD). The psychosis may be due, in part, to overstimulation of central serotonergic (5-HT) receptors. We treated 16 PD patients who had psychosis of 6 to 60 months' duration with the 5-HT3 receptor antagonist ondansetron (12 to 24 mg daily) in an open-label, short-term (4 to 8 weeks) trial. There was marked to moderate improvement (p < 0.01) in measures of visual hallucinations, paranoid delusions, confusion, and the associated global functional impairment in all but one of the patients, and there was moderate improvement in the Brief Psychiatric Rating Scale and the Nurse's Observation Scale for Inpatient Evaluation; the Mini-Mental State Examination scores remained unaltered. Ondansetron did not cause any worsening in basic PD symptoms or levodopa efficacy and was well tolerated with no major side effects. Our study suggests that pharmacologic blockade of central 5-HT receptors may become a strategy to attenuate PD psychosis without inducing motor deterioration or suppression of antiparkinsonian action of levodopa, and it lends support to the hypothesis that serotonergic mechanisms are pathogenetically important in the emergence of psychosis in PD.