Non-bleeding visible vessel and sentinel clot are terms used interchangeably to describe protuberances in the base of ulcers that have recently bled, but a consensus as to their definition or natural history does not exist. In patients with severe ulcer hemorrhage, non-bleeding protuberances were classified as vessels, with or without a small attached clot, or as sentinel clots, according to a schema based on the appearance of the protuberance at endoscopy but not subjected to pathologic correlation. Endoscopic therapy was not performed at the index endoscopic evaluation, and natural evolution was prospectively documented with daily videoendoscopy. Eleven (46%) of 24 patients with non-bleeding protuberances had rebleeding. Independent classification by three authors concurred in 18 (75%) of 24 lesions. Ten (91%) of 11 vessels with or without attached clot rebled versus 0 (0%) of 7 sentinel clots and 1(17%) of 6 lesions without unanimous classification (p < 0.01, vessels versus other groups). Rebleeding occurred in 5 (71%) of 7 nonpigmented (pale or white), 6 (38%) of 16 red or purple, and 0 (0%) of 1 black protuberances. In general, vessels persisted until rebleeding, whereas sentinel clots disappeared within 1 to 3 days. We conclude that nonbleeding protuberances in ulcer bases can be separated into vessels, which have a high risk of rebleeding, and sentinel clots, which have a low risk of rebleeding.