There has been much concern among the public and veterans that specific environmental exposures incurred during the Gulf War were the cause of subsequent illness among Gulf War veterans. In this historical cohort study, the authors compared the postwar morbidity of US military personnel exposed to smoke from the 1991 Kuwaiti oil well fires with that of unexposed personnel. Complete exposure and demographic data were available for 405,142 active-duty Gulf War veterans who did not remain in the region after the war. The authors used data from all Department of Defense hospitals for the period August 1, 1991-July 31, 1999 to estimate rates of hospitalization due to any cause, hospitalization due to a diagnosis in one of 15 major categories, and hospitalization due to one of nine diagnoses likely to be manifestations of smoke exposure. Exposures to particulate matter from oil-well-fire smoke were based on the integration of meteorologic data, diffusion modeling, and troop location data. The authors constructed seven exposure groups combining duration and amount of exposure. In Cox modeling, three of the 25 models showed an increased adjusted risk of hospitalization. However, there was no evidence of a dose-response relation. Despite some limitations, these data do not support the hypothesis that Gulf War veterans have an increased risk of postwar morbidity from exposure to Kuwaiti oil-well-fire smoke.