Obesity is often associated with poor heat tolerance. This case-control study was to determine the effect of obesity measured by the Body Mass Index (BMI) on the occurrence of heat disorders. The subjects, 218 soldiers with heat disorders and 537 controls, were matched for age and sex. Obesity was defined as a BMI of greater than 27. The odds ratio for obese soldiers was 3.53; however, their rectal temperatures were not significantly higher at presentation for heat disorder. The Physical Employment Standard (PES) classifies soldiers into A, B, C, and E depending on medical history at the time of enlistment. Soldiers with PES status of A and B were at a marginally higher risk of heat disorders. Correcting for the effect of the PES status, the odds ratio for obese soldiers was 4.29. Therefore, obese soldiers training in a hot and humid environment are at an increased risk of heat disorders.