For more than a decade after the Gulf War, there has been concern that wartime exposures have resulted in significant morbidity among Gulf War veterans. After the end of the war, the Department of Veterans Affairs (VA) and the Department of Defense (DoD) initiated health registries to provide systematic clinical evaluations of Gulf War veterans who chose to participate. By September 1999, there were 32,876 participants in the DoD Comprehensive Clinical Evaluation Program and 70,385 participants in the VA Gulf War Registry Health Examination Program. We identified demographic and military service factors, as well as potential war-related exposures associated with subsequent registry participation after 10 years of observation. Veterans potentially exposed to oil well fire smoke, those near Khamisiyah, Reserve and National Guard, Army veterans, and veterans in the theater of operations during intense combat periods were most likely to elect to participate in a registry. These findings support the hypothesis that certain occupational factors and wartime exposures may influence subsequent health care-seeking behavior.