The study population was defined as all active duty military and mobilized reserves and National Guard who served for at least one month at any time during the Gulf War period. Mobilized reservists and members of the National Guard were those who were activated and called-up, being either deployed to S.W. Asia or "back-filling" positions of military personnel who were deployed. Deployed military personnel were those who served in the S.W. Asian theater during Operations Desert Shield and Desert Storm or the period immediately after Desert Storm or received hazardous duty pay during the Gulf War. All other individuals in the study population were classified as non-deployed, although they may have been stationed outside the U.S.