On 13 April 1982, after a 6 to 7 km training run on an extremely hot and humid day, 20 of 216 cadets at the Ecuadorian Naval Academy in Guayaquil became ill with symptoms of heat-related illness. Four of them suffered heat stroke, and three died. Illness was most highly associated with the amount of exercise performed that day but was significantly more common in first-year cadets than in older cadets. There were no differences in the anthropometric measurements of ill and well subjects. We conclude that heat-related illness can affect conditioned military personnel during routine training in severe weather and make recommendations for prevention.