To assess renal function changes induced by marathon racing in asymptomatic runners, we studied them before, six hours after, and one week after warm and cold weather marathons (26.2 miles [42 km]). Standard serum electrolytes, creatine phosphokinase (CPK), urinalysis, urinary myoglobin, and renal function tests (para-aminohippurate [PAH], inulin, and true creatinine clearances) were performed. After rehydration to their prerace weight, the subjects showed no postrace change in serum electrolytes. The CPK rose postrace and was two to three times higher in the warm weather runners than in the cold weather runners. All postrace urinalyses were grossly abnormal. Urinary myoglobins were positive postrace in warm weather runners and negative in cold weather runners. Warm weather runners showed a 50% decline in inulin clearance postrace but maintained PAH clearance. At one week, inulin clearance returned to baseline but fractional excretion of creatinine was below unity. In contrast, cold weather runners showed no change in inulin and PAH clearances postrace or at one week, but fractional excretion or creatinine postrace was less than unity returning to baseline at one week. We conclude that renal function abnormalities occur in marathon runners and that the severity of the abnormality is temperature-dependent.