Exercise-induced hyponatremia is commonly believed to be associated only with extraordinary physical efforts, or particularly strenuous exercise. Hyponatremia complicating moderate exercise has not been described previously. The authors describe the characteristics of seven patients with life-threatening hyponatremia associated with mild to moderate exercise. All patients suffered from nausea, vomiting, agitation, and confusion, appearing during or after moderate physical activity. Grand mal convulsions occurred in five of the patients. In laboratory results, hyponatremia was as low as 115 mEq/L, with a relatively high sodium concentration in the urine. High serum creatine kinase activity levels were found in most of the patients. All patients were discharged in good condition, without neurologic sequela. The authors conclude that hyponatremia is a possible complication of moderate exercise, and not only of endurance sports, and that exercise-induced hyponatremia can produce severe neurologic manifestations. The mechanism of the hyponatremia is unclear, but may be due to a hemodynamically inappropriate stimulus for antidiuretic hormone secretion.