Case reports have highlighted angiogenic polypoid proliferation in the mucosa adjacent to ileal carcinoid tumors, describing them as granulation tissue polyposis and florid angiogenesis. Some authors have proposed that the ileal carcinoid tumors themselves produce growth factors that cause the change. The purpose of this study was to determine the frequency of angiogenic polypoid proliferation in a large cohort of resected ileal carcinoid tumors compared with control groups. Search of the Cleveland Clinic and Summa Health System pathology files (1985 to 1999) yielded 65 resected ileal carcinoid tumors. Mucosal abnormalities adjacent to the ileal carcinoid tumors were graded 0 to 4+. Twenty ileal resection margins from colonic carcinoma cases served as normal controls. Ileal mucosa adjacent to 22 noncarcinoid neoplasms were also examined. The mucosa adjacent to 54/65 ileal carcinoid tumors (83%) showed mucosal abnormalities (vs. 3/20 normal controls), including mucosal edema, capillary ectasia, muscularis mucosae hypertrophy, fibrosis/smooth muscle proliferation within the lamina propria, club-shaped villi, and intramucosal capillary proliferation. Forty ileal carcinoid tumor cases (61%) showed some degree of angiogenic polypoid proliferation characterized by club-shaped villi and prominent intramucosal capillaries, with 17 (26%) graded as 3+ or 4+. Angiogenic polypoid proliferation was associated with hypertrophy of the muscularis mucosae, lamina proprial fibrosis/smooth muscle proliferation, and capillary ectasia similar to that described with gastrointestinal mucosal trauma/prolapse. This trauma/prolapse change was identified in 45 cases (69%) and was graded 3+ or 4+ in 23 (35%). Seventeen (77%) of the noncarcinoid neoplasms showed trauma/prolapse changes, with 7 (32%) graded as 3+ or 4+. Angiogenic polypoid proliferation also correlated with trauma/prolapse change in the noncarcinoid neoplasm controls. Neither APP (P =.24) nor the prolapse changes (P =.33) were found to be statistically different between the two tumor groups. Angiogenic polypoid proliferation of the adjacent ileal mucosa is common in patients with ileal carcinoid tumors and with noncarcinoid neoplasms. Angiogenic polypoid proliferation almost invariably coexists with fibromuscular change and capillary ectasia within the lamina propria, suggesting that mucosal trauma/prolapse plays a role in the histogenesis. The association of angiogenic polypoid proliferation with a variety of different neoplasms makes it unlikely that the tumors themselves secrete growth factors.