A study of 13 men with grossly bloody urine induced by running incriminated the urinary bladder neck or posterior urethra as the site of blood loss. Our evidence also showed that diet lacking in animal protein can play a causative role. The ailment seems to have no long-term sequelae and may not affect female runners. Although we recommend medical evaluation for any runner with gross hematuria, the extent of diagnostic work-up and any modification of the victim's training problem program will depend on the circumstances in each case.