Background and Study Aims: It is not normally difficult to diagnose carcinoid tumors (well-differentiated endocrine neoplasms) of the rectum endoscopically, as they usually have a characteristic appearance. However, little is known about the atypical endoscopic findings in some rectal carcinoids and the present study was performed to analyze these. Patients and Methods: The endoscopic findings in 67 consecutive patients with rectal carcinoids (37 men, 30 women; age range 23-76) were analyzed retrospectively. Results: Tumor size ranged from 2 mm to 30 mm (average 7.4 mm). Of the 67 patients, 52 (78%) displayed the characteristic endoscopic findings of smooth, round, sessile elevations covered with normal-appearing or yellow-discolored mucosa; in 15 (22%) there were one or more atypical endoscopic findings. These included a semipedunculated appearance (n = 6), hyperemia (n = 5), a central depression (n = 6), erosion (n = 5), and ulceration (n = 4). Atypical findings were noted in none of 20 carcinoids < 5 mm in diameter; in six (20%) of the 30 carcinoids between 5 mm and 9 mm; in six (43%) of the 14 carcinoids between 10 mm and 19 mm; and in three (100%) of the three carcinoids ≥20 mm in diameter (P< 0.001). Invasion into the muscularis propria or metastasis to the liver or lymph nodes occurred in three of the four patients with ulceration, but it was confirmed in only one of the 63 patients without ulceration (P<0.001). Conclusions: Atypical endoscopic appearances of rectal carcinoids are observed more frequently as the size of the tumor increases and a finding of ulceration may have a prognostic value.