The Comrades Marathon is a gruelling test of man's stamina and only trained athletes participate. It is surprising that of over 2,000 contestants each year on a 90-km (56.25-mile) course, severe cardiac and renal damage does not occur more often. We are able, however, to report 10 cases over 9 years when renal damage did occur. 3 cases required haemodialysis; 1 had peritoneal dialysis; 2 patients had renal biopsies. Possible idiosyncratic factors are discussed as is the pathogenesis of acute tubular necrosis.