In patients admitted to a general medical ward, the incidence of hyponatremia on admission was 3.6%. Mortality was more than three times higher among patients with hyponatremia than among the patients as a whole. The triad hyponatremia, hypoalbuminemia and hypervolemia indicated a poor prognosis. A diversity of medical illnesses was associated with hyponatremia, and the use of diuretics was a major contributor to the electrolyte disturbance. Treatment of asymptomatic hyponatremia is discussed, with special emphasis on restriction of fluids.