In ulcerative colitis (UC), epithelial proliferation plays a role in crypt repair and neoplastic evolution. Proliferative status is predominantly connoted in active disease, but not defined in remission. Histologically, remission is characterized by normalization of the picture or development of atrophy. Mutation of the ras oncogene is involved in intestinal carcinogenesis. Aim of this work was to assess the proliferative pattern of rectal epithelium in UC during disease activity and in remission and correlate it with ras oncoprotein p21. The study was performed retrospectively in rectal biopsies from four groups each of 10 patients: active ulcerative colitis (AUC), remission with a normal histology (RUC), remission with rectal atrophy (ARUC), and irritable bowel syndrome (C, control group). In all, immunohistostain was employed to evaluate the proliferation cell nuclear antigen labeling index (PCNA LI) and ras p21. Statistical analysis was performed by ANOVA and Student-Neumann-Keuls tests. PCNA LI was significantly higher in AUC and ARUC than in RUC and C. Positive cells were predominant in the lower zone of crypts in RUC and C, while a significant expression of PCNA was also observed in the upper areas in AUC and ARUC. Oncoprotein p21 was expressed on the apical surface of the epithelium in 3/10 AUC patients, in all 10 ARUC patients and in none of RUC and C. The persistently increased epithelial proliferation associated with ras p21 expression in ARUC may be due to the action of an abnormal, mutated ras gene that could play a role in UC-related tumorigenesis.