Update: Exertional hyponatremia, active component, U.S. Armed Forces, 1999-2011.

From 1999 through 2011, there were 1,329 incident diagnoses of exertional hyponatremia among active component members. In 2011, there were more incident cases (n=183) than in any other year of the period and more than three-times as many cases as in 1999 (n=57). Crude incidence rates of exertional hyponatremia were highest among females, Marines, and those in the youngest and oldest age groups. The increased incidence in the U.S. military overall reflects sharply increasing rates in the Marine Corps and slight increases in the other services, particularly since 2005. Service members (particularly trainees and women) and their supervisors must be vigilant for early signs of heat-related illnesses and must be knowledgeable of the dangers of excessive water consumption and the prescribed limits for water intake during prolonged physical activity--e.g., field training exercises, personal fitness training, recreational activities--in hot, humid weather.