The occurrence of hematuria as a short-lived and occasional event in a number of long-distance runners appears to be secondary to bladder contusions caused by a combination of exertional forces and intraabdominal pressure producing repeated impact of the flaccid wall of the bladder against the bladder base. The bladder may be empty or nearly empty while the runner is in action. This permits apposition of the surfaces of the bladder. The variability of the state of filling of one's bladder from one occasion to another may therefore account for the inconsistency of occurrence of hematuria in the same runner.