A serum sodium concentration of 110 mmol/L or less is thought to be extremely dangerous; mortality rates of 33% to 86% have been cited. Experience at the University of Rochester School of Medicine does not corroborate this widespread belief. Among 62 patients with 64 episodes of severe hyponatremia, the mortality rate was only 8%, and most deaths were caused by underlying diseases; with a serum sodium concentration of 105 mmol/L or less, the mortality rate was 5%. Rapid correction of hyponatremia did not account for this favorable prognosis. Only 6% of the 64 cases were corrected to a serum sodium concentration of 120 mmol/L at recommended rates of 1.5 to 2 mmol/(L.h); half of the patients achieved this rate of correction because of a spontaneous water diuresis. In some settings, vigorous therapeutic efforts may have contributed to an adverse outcome. Rapid correction of hyponatremia is not indicated just because the serum sodium concentration is extremely low.