Hydrogen peroxide is a widely available disinfectant that has been reported to cause colitis. We report a case of a 67-year-old man who presented with an acute proctitis caused by a self-inflicted 3% hydrogen peroxide enema. The patient's intention was to cure himself of a recently diagnosed prostate cancer, because the waiting list for oncological consultation was deemed too long. The pathogenesis of hydrogen peroxide mucosal injury and a review of the literature is discussed.