BACKGROUND/AIMS: Clinical outcomes and factors predictive of favorable response after 5-aminosalicylates or sulfasalazine (5-ASA/sulfasalazine) treatment alone have not been well established in the treatment of mild to moderate ulcerative colitis (UC). We evaluated the clinical course of Korean UC patients treated with 5-ASA/sulfasalazine as a maintenance therapy in terms of relapse and predictive factors of clinical relapse. METHODOLOGY: A total 256 UC patients, treated with 5-ASA/sulfasalazine at the Severance Hospital between January 2000 and December 2008, were analyzed retrospectively. We sought to investigate relapse rates and to determine independent predictors for relapse. RESULTS: Of the 256 patients, 127 patients (49.6%) had a disease relapse. The cumulative relapse rate was 21.5% after 1 year, 36.5% after 2 years, 46.9% after 3 years and 59.8% after 5 years. On multivariate analysis, left-sided or extensive colitis at diagnosis (hazard ratio=1.46; 95% CI=1.01-2.10; p=0.04) and initial hemoglobin level <10.5g/dL (hazard ratio= 0.43; 95% CI=0.22-0.81; p=0.01) were found to be independent factors for clinical relapse. CONCLUSIONS: Our study showed that both disease extent at diagnosis and anemia were major predictive factors for clinical relapse after 5-ASA/sulfasalazine therapy for Korean patients with mild to moderate UC.