Classic teaching emphasizes that chronic ulcerative colitis is characterized morphologically by the presence of fixed architectural and cellular mucosal changes that categorize the process as chronic. To examine the effect of topical 5-aminosalicylic acid (5-ASA) enemas on the presence of six histological features of chronicity in established chronic ulcerative colitis, 123 mucosal biopsies were taken prospectively at 1-month intervals, all from the same anatomic location (10 cm), from 14 patients treated with either 5-ASA or placebo enemas. The biopsies were evaluated for the presence of mixed inflammation in the lamina propria, crypt architectural abnormalities, basally located lymphoid aggregates, basal plasmacytosis, villiform surface epithelial configuration, and Paneth cell metaplasia. Overall, 29% of biopsies from 64% of patients were histologically normal (no chronic features, no active disease). Compared with patients treated with placebo enemas, patients treated with 5-ASA enemas showed a significantly higher percentage of normal biopsies (36% ASA group vs. 12% placebo group; p = 0.005) and a lower percentage occurrence of each individual histological feature of chronicity. In addition, patients treated with 5-ASA had a higher average number of normal biopsies per patient (3.0) than those treated with placebo enemas (1.3). Therefore, histologically normal-appearing mucosal biopsies do occur in established cases of chronic ulcerative colitis, and this finding is enhanced by treatment with 5-ASA enemas. Awareness of these results should prevent the presence of normal rectal mucosal biopsy findings in chronic ulcerative colitis patients from being misinterpreted as either evidence against this diagnosis or as representing focal skip areas characteristic of Crohn's disease.