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. 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. 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. 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.