A woman with a history of psychiatric illness is brought into the emergency ward of a hospital after the occurrence of several insults and overt signs of coma. Laboratory investigation shows a severe hyponatraemia. A diagnosis of primary polydipsia seems probable. After an absolute waterrestriction of 24 hours both consciousness and the sodium concentratium in plasma have normalised. Review of both the psychiatric and somatic history of the woman reveals that she may be classified as the prototype of a patient with primary polydipsia.