This 9-year-old boy with a history of behavioral problems and worsening psychosis responded initially to treatment with haloperidol. However, a magnetic resonance image obtained as part of his psychiatric evaluation revealed an anterior third ventricle tumor and mild-to-moderate hydrocephalus. The resected tumor was found on pathological examination to be a choroid plexus papilloma. The patient had an uneventful postoperative course and remained free of psychosis or mood disorder at 1-year follow-up examination.