We report the case of a physically fit young man who presented with severe, life-threatening heat stroke after running a 10 km road race. He had previously received bilateral thoracic sympathectomy for axillary hyperhidrosis at another hospital, and was known to have upper body and limb anhidrosis. Thoracic sympathectomy can result in reduced sweating and disturbed peripheral vascular and heart rate responses. Patients should be warned that these mechanisms may play a role in the development of exertional heat stroke.