Studies have shown that rectal sparing and patchiness develop in treated and longstanding ulcerative colitis (UC), making the distinction from Crohn colitis increasingly difficult after treatment is initiated. However, no histologic studies of the incidence of rectal sparing in adults at UC onset have been performed. Colectomy specimens from 46 patients with classic UC histologic features and no Crohn disease features were identified. Biopsy specimens obtained before medical therapy were retrieved and examined blindly by 2 pathologists, along with appropriate control samples. Slides were scored for chronicity (crypt branching, subcryptal plasma cells, lamina propria plasma cells) and activity (cryptitis, crypt abscesses, epithelial injury). In 28 cases, only rectal biopsy specimens were taken; for 16, rectal and at least 1 proximal biopsy specimen were taken. All cases showed rectal involvement; none had rectal sparing at initial biopsy. Of 16 cases with rectal and more proximal biopsy specimens, 5 (31%) showed relative rectal sparing (lower scores in rectum than in more proximal sites). In 16 cases with rectal and more proximal biopsy specimens, chronicity and activity scores were higher in the rectum than in more proximal sites (P = .01; chronicity and activity). The mean overall chronicity score decreased in a linear manner from rectum to cecum. The rectum is involved and shows evidence of chronicity and activity at disease onset in UC, using colectomy as the gold standard for diagnosis. Because rectal sparing at UC onset has been reported, a prospective study using uniform biopsy protocols is needed to establish the true incidence of rectal sparing at presentation.