The urine of 45 athletes, male and female, were studied after completing an ultra long-distance marathon race. Of these athletes, 24.4% were found on screening to have red blood cells in the urine. The urinary mean corpuscular volume (MCV) of red cells was used in an attempt to differentiate between lower and upper urinary tract haematuria. With the exception of 1 athlete, the red cells were not dysmorphic and had an MCV greater than 72 fl. This suggested that the haematuria had an origin from the lower urinary tract. The urinary changes disappeared within 7 days. No definite residual renal damage could be established.