Of an estimated 17 632 participants of the 1979 Peachtree Road Race, a 10-km race held at 0800 on 4 July in Atlanta, Georgia, 29 had severe heat injury (collapse with altered mental status and rectal temperature of 39.7 °C or greater); all 29 recovered promptly. The temperature was 24.0 °C; the relative humidity, 83%; and the wet bulb-globe index, 23.0 °C. Compared with unaffected participants matched by age, sex, and predicted finishing time, case-runners were taller, had slower best recent 1.6-km and 10-km times, had run less in June, were more likely to achieve 89% of estimated maximum aerobic capacity during the race, less often splashed with water, were more likely to have underestimated their predicted finishing times based on their best recent 10-km times, and were more likely to live in Georgia. Height, sprinkler use, and underestimating predicted finishing times were each significant in the presence of the others. These observations suggest that participants of similar races, particularly those taller than 179.2 cm, should splash with water at least twice and base their predicted finishing times on their best recent 10-km times; adherence to these recommendations by runners at risk should reduce their severe heat injury risk at least 8.1 times.