A relationship between acute psychosis, water ingestion, and the syndrome of inappropriate secretion of antidiuretic hormone (SIADH) has been reported. This syndrome was observed in a psychotic patient who ingested massive amounts of water and became symptomatically hyponatremic with seizures. Although he had been taking haloperidol, the SIADH responded to fluid restriction alone. SIADH was clearly established, and a temporal relationship to his acute exacerbation of psychosis and polydipsia could be shown. This case illustrates that psychosis, polydipsia, and SIADH are often associated as a triad, and that psychiatric history must be considered in the evaluation of this syndrome.