The risk of heat exhaustion at a deep underground metalliferous mine was assessed in relation to the body-mass index (BMI) and predicted maximal oxygen uptake (V̇O₂max) of miners, using case-control methodology. Sixty-five cases of acute heat exhaustion and 119 controls were studied. Heat exhaustion cases had a significantly higher BMI than controls (P=0.006). The odds ratios increased with BMI. For a BMI of 32.00-36.99, compared to a BMI of less than 27.00 the odds ratio was 3.63 (95% confidence interval, 1.42-9.36). V̇O₂max was not significantly lower in cases than controls. The odds ratios for heat exhaustion increased with decreasing V̇O₂max, but not significantly. The sample size provided 80% power of detecting an odds ratio of 2.5 or greater. Deep underground miners should be advised to maintain a BMI of 24-27. Selection of miners on the basis of BMI should not be used as an alternative to satisfactory engineering controls such as ventilation and refrigeration.