Update: Exertional hyponatremia, active component, U.S. Armed Forces, 1999-2012.

From 1999 through 2012, there were 1,333 incident diagnoses of exertional hyponatremia among active component members of the U.S. Armed Forces. Annual incidence rates rose sharply from 2008 to 2010 but have since decreased by 50 percent from 2010 to 2012. In 2012, there were fewer incident cases (n=84) than in any of the previous six years. The recent decrease in rates overall reflects sharply declining rates in the Marine Corps and slight decreases in the other services. Relative to their respective counterparts, crude incidence rates of exertional hyponatremia for the entire 14 year surveillance period were higher among females, those in the youngest age group, Marines, recruit trainees, and "other" military occupations. 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.