A cross-sectional survey of the drinking habits of 877 mentally handicapped in-patients revealed 31 patients (prevalence 3.5%) who, in the opinion of nurses, drank five litres or more daily. Low urine specific gravity was a less useful indicator of polydipsia. Polydipsia appeared to be significantly associated with a borderline level of handicap and with a diagnosis of schizophrenia, autism or severe personality/behaviour disorder. Of five cases of water intoxication associated with polydipsia, one was fatal. In two cases excess drinking improved with increased neuroleptic medication. Lithium and demeclocycline were used in two cases to prevent hyponatraemic episodes.