Between 1972 and 1979 34 patients with severe hypernatraemia (serum sodium larger than or equal to 160 mmol/l) were admitted to the intensive care unit. Among these there were 45% of neuropsychiatric patients with reduced consciousness. Maximal serum sodium concentration was 168 (160--204) mmol/l, maximum serum osmolality 391 (340--520) mosm/kg H2O. Treatment consisted of infusion of hypotonic solutions and potassium substitution. Fluid balance was positive (5,9 during 43 hours) until normal serum sodium concentrations had been reached. The mortality of 77% was exclusively due to the underlying diseases.