A matched population-based case-control study was conducted on exertional heat illness (EHI) in male Marine Corps recruits in basic training at Parris Island, SC. Physical fitness and anthropometric measurements were obtained for 391 of 528 cases of EHI identified in this population during 1988-1992, and 1467 of 1725 controls matched to cases by initial training platoon. The risk for developing EHI increased with increase in body mass index (BMI = weight·height-2) as measured on arrival and with increase in time to complete a 1.5-mile run conducted during the first week. Recruits at highest risk for developing exertional heat illness had a BMI of 22 or more kg·m-2 and a 1.5-mile run-time of 12 or more minutes. These recruits had an eightfold higher risk for developing exertional heat illness during basic training when compared with those with BMI less than 22 kg·m-2 and 1.5-mile run-time under 10 min (P < 10-6). Only one-fifth (18%) of male recruits met these criteria for high risk, but they accounted for nearly half (47%) of the exertional heat illness cases occurring during the 12-wk basic training course.