Training regimens and race days place significant demands upon both the competitive endurance athlete and the frequent-recreational runner. Lower gastrointestinal derangements, especially those involving diarrhea and rectal bleeding, are common and can impact adversely both the performance and the health of the athlete. While most cases are relatively benign, more significant and severe symptoms may not only impair sports performance, but also signify more serious disease. The sports medicine clinician should be familiar with the management of these problems in order to optimize treatment, facilitate return to play, and maximize the athlete's potential.