AIM: The aim of this study is to analyse the clinical course of ulcerative colitis during maintenance therapy with azathioprine, a drug which is still not proved to be able to modify the natural history of the disease. METHODS: A retrospective study is made of data regarding the frequency of hospital admission for patients with ulcerative colitis referring to a gastroenterological Day-Hospital between 1991 and 2000. The disease history of these patients has been divided into 2 sections: one preceding and the other following an index-event, identified as the beginning of a maintenance regimen with azathioprine; this allowed to find possible differences in the clinical course after the index-event. Patients were controls of themselves. RESULTS: Seventeen patients qualified for analysis. Remission from an acute severe attack of ulcerative colitis was reached by intravenous or oral cyclosporine for 14 of them and by prednisone for 3 of them. The maintenance treatment with azathioprine, which started in all but 1 patient (intention-to-treat), showed a reduction in the number of hospital admissions, decreasing from a mean of 2.12+/-0.69 in the preceding 4.2+/-4.3 years to a mean of 0.12+/-0.33 in the following 5.8+/-2.5 years (p=0.000). CONCLUSIONS: Patients undergoing maintenance therapy with azathioprine showed face fewer relapses needing hospitalisation than those without azathioprine.