OBJECTIVE This study assesses the epithelial cell proliferation in different degrees of diverticular disease and compares this with two different control groups. BACKGROUND Hyperproliferation of the colonic mucosa has been recently described in diverticular disease (DD), but it is unknown whether this finding is present in every degree of DD. METHODS Thirty consecutive patients with a new endoscopic diagnosis of DD (10 with asymptomatic diverticulosis, 10 with symptomatic uncomplicated DD, and 10 with acute uncomplicated diverticulitis) were enrolled. Ten matched healthy people and 10 patients with ulcerative colitis (UC) (5 with UC in remission and 5 with active UC) were enrolled as control groups. The Ki-67 antigen index of the whole crypt and the upper third was separately evaluated. RESULTS Ki-67 index of the whole crypt and the upper third of the crypt was significantly higher in all degrees of DD compared with the healthy control group. In particular, asymptomatic diverticulosis showed a threefold higher Ki-67 index compared with that of the healthy control group (5.4% and 5.6% vs. 1.6% and 1.8%, respectively, P = 0.005), and similar to that of UC in remission (5.4% and 5.6% vs. 5.9% and 5.8%, respectively, P = not significant). CONCLUSIONS We found an upward shifting of cellular proliferation of the colonic mucosa in patients with different degrees of DD. In particular, asymptomatic diverticulosis seems to show the same risk for colonic carcinoma as that of UC.