Cavernous hemangiomas are rare lesions of the colon that usually present with painless, recurrent bleeding. Hemangiomas can be capillary or cavernous type, and 80% of rectal hemangiomas are cavernous type. Ulcerative colitis is an inflammatory disorder that affects the rectum and, occasionally, the whole colon. Diarrhea, rectal bleeding and mucous discharge characterize ulcerative colitis. We present a 61-year-old man with painless rectal bleeding due to a solitary cavernous hemangioma of the rectum. He had been diagnosed with distal type ulcerative colitis in 2003. He was asymptomatic under, mesalazine treatment until May 2005, when he presented with new onset bright red rectal bleeding and mucous discharge, despite still defecating normal stools once a day. Rectosigmoidoscopic examination revealed mucosal hyperemia, edema, granularity, and a hyperemic, friable mass lesion 5x4 cm in diameter in the rectum. Following excision, histopathologic examination of the mass was consistent with cavernous hemangioma. There was a six-month period between the rectosigmoidoscopy in which the cavernous hemangioma (5x4 cm. in diameter) was detected and the former rectosigmoidoscopy with no reported hemangioma. Thus, this was considered a rapidly growing cavernous hemangioma. Intralesional microhemorrhages may cause rapid enlargement of the hemangiomas. Ulcerative colitis is characterized by inflammation, which may interfere with vascular integrity and augment intralesional microhemorrhage. We postulate that the inflammatory background of ulcerative colitis may have accelerated intralesional hemorrhage and growth of this coincidental rectal cavernous hemangioma. To the best of our knowledge, this is the only case of this sort in the literature.