Study Abstract: Recently, concern has arisen regarding a possible elevated occurrence of ALS among veterans who served in the Persian Gulf during Operations Desert Shield (August 2, 1990 - January 15, 1991), Desert Storm (January 16, 1991 - February 28, 1991) and Clean-up (March 1, 1991 - July 31, 1991). In response to this concern, a nationwide epidemiological investigation was conducted to ascertain all occurrences of ALS between August, 1990 and July, 2000 among all active duty military and mobilized Reserves, including National Guard, who served during the Gulf War period (August 2, 1990-July 31, 1991). A diagnosis of ALS was confirmed by medical record review and, as necessary, neurological examination. Risk was assessed by the age-adjusted, average, annual 10-year cumulative incidence rate. VA led this joint federal government epidemiologic study that also involved DoD, HHS, CDC, and academic centers of excellence in neurology, with advice from the ALS Association.