It has been suggested that colon cancer risk in ulcerative colitis (UC) is correlated to a reduced bioavailability of folate. We studied the effects of folate supplementation on the pattern of rectal cell proliferation in patients affected by long-standing UC. In the rectal mucosa of these patients, an expansion of proliferating cells to the crypt surface is found frequently. This abnormality is considered an intermediate biomarker in chemoprevention trials. Twenty-four patients (13 males; age, 26-70 years; UC duration, 7-34 years) with UC in remission for 1 month at least were assigned randomly to one of the following treatments: (a) folinic acid (15 mg/day) or (b) placebo. Cell proliferation was analyzed through immunohistochemistry on sections of rectal biopsies incubated for 1 hour in a culture medium containing bromodeoxyuridine. Fragments were taken at admission to the study and after 3 months of treatment. As compared to the baseline values, after 3 months of therapy in patients treated with folinic acid, a significant reduction of the frequency of occurrence of labeled cells in the upper 40% of the crypts (phi h value) was observed (0.1836 +/- 0.0278 versus 0.1023 +/- 0.0255; P < 0.01). On the contrary, no significant proliferative changes were observed in the placebo group. These results suggest that folate supplementation contributes to regulating rectal cell proliferation in patients with long-standing UC. These findings may be significant for chemoprevention of colon cancer in these patients.