Four cases of symptomatic hyponatraemia in psychiatric patients are described. All patients exhibited varying degrees of the compulsive intake of water. The initial urinary osmolalities in all cases suggested impaired clearance of free water. This may be related to an exacerbation of the psychotic state. All patients demonstrated a marked diuresis after treatment with a hypertonic (1.8%) saline solution. The underlying pathophysiology is discussed. This clinical syndrome may be associated with significant morbidity and occasional mortality that are related to the rapid development of hyponatraemia.