OBJECTIVE To determine whether caregiver responses to a screening question are a reliable method of identifying polydipsia (excessive water drinking) in institutionalized residents with mental retardation. To review the etiology, acute and chronic clinical manifestations, and care of polydipsia and water intoxication. METHODS This paper presents an assessment of interrater reliability for a screening question for polydipsia using responses of primary caregivers of preidentified polydipsia cases (n = 32) and matched controls (n = 33) in a large Canadian institution for developmentally handicapped adults. A chart review of all cases of identified water intoxication is also provided. The behavioural outcomes of preventive measures for water intoxication are described. RESULTS The screening instrument was reliable, having a kappa (interrater reliability) of 0.73. Several case descriptions illustrate typical presentations of water intoxication in this population. CONCLUSIONS Polydipsia is reliably identified by caregiver responses to a screening question. It should be screened for regularly to ensure appropriate care to prevent important acute and chronic complications.