Hyponatremia (severe sodium depletion) has symptoms similar to heat exhaustion and heat stroke and can easily be misdiagnosed. The number of wilderness users and extreme adventure activities has increased in recent years, and more cases are being diagnosed. Given that a 1993 study found that 1 in 10 cases of heat-related illnesses were hyponatremia, wilderness medicine providers need to become familiar with this illness. In hyponatremia, as sodium decreases in relation to water, water moves from the cardiovascular system into the intracellular space in the brain, which can result in cerebral edema followed by seizures, coma, and death. Symptoms of hyponatremia that differentiate it from heat exhaustion and heat stroke are decreased thirst, increased urine output and frequency of urination, clarity of urine, lack of poor skin turgor (tenting of the skin), and moist mucous membranes. Patients with hyponatremia may have a significant decrease in level of consciousness, but will not have hot skin or a high core body temperature. Hyponatremia often occurs after the strenuous activity has ceased, while heat exhaustion and heat stroke occur during the activity. Knowledge of patient activity level and duration, food and water intake, medical history, and medications are extremely important. Treatment of hyponatremia in the wilderness includes rest, maintaining a stable core body temperature, withholding fluids, and increasing sodium intake (in conscious patients). Two charts present an algorithm for the assessment of heat illnesses and a comparison of symptoms for heat exhaustion, heat stroke, and hyponatremia. (Contains 11 references.) (TD)