Update: Exertional hyponatremia, active component, U.S. Army, Navy, Air Force, and Marine Corps, 2000-2015.

From 2000 through 2015, there were 1,542 incident diagnoses of exertional hyponatremia among active component members of the U.S. Army, Navy, Air Force, and Marine Corps. Annual incidence rates rose sharply from 2008 through 2010 but then decreased by more than 50% from 2010 through 2013. In 2015, the number of cases (n=116) increased by approximately 20% from the previous year. The recent increase in rates overall reflects increased rates in the Army and the Marine Corps. Relative to their respective counterparts, crude incidence rates of exertional hyponatremia for the entire 16-year surveillance period were higher among females, those in the youngest age group, Marines, and recruit trainees. Service members (particularly recruit trainees) 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.