Abstract Background The clinical course of ulcerative colitis (UC) is characterized by periods of exacerbation and remission. 5-aminosalicylic acid (5-ASA) remains the treatment of choice in patients with mild-to-moderate UC, due to its beneficial effect in preventing relapse. Treatment options (immunomodulators [IMM] and biological agents) have increased, so it is important recognizing patients at risk of medical therapy failure that can appropriately optimize treatment escalation for better control of the disease. Little is known about the long-term course of the disease among patients in remission with 5-ASA. Aims To assess long-term efficacy of 5-ASA (no IMM, biological therapy or colectomy) and to identify predictive factors of non-response. Methods An observational, retrospective, single-centre, case-control study was performed. All consecutive patients with UC followed in our Hospital from January 2000 until June 2020 initially treated with 5-ASA (±steroids) with a minimum follow-up of 1 year were screened for eligibility. Exclusion criteria were 5-ASA toxicity/intolerance or initial treatment with IMM/biologic agents.Time until step-up was recorded. Demographic, clinical, endoscopic, biochemical, and histological variables at the diagnosis were recorded and a Cox regression was performed to identify predictive factors for step-up strategy. Results 218 patients (mean age 41,4±14,6 years; 54,5% female; mean follow-up10,1±1,9 years) were analysed, 28% needed treatment with biologics or IMM, mainly due to steroid-dependence. Among “5-ASA responders”, 72% and 48% had sustained endoscopic and histological remission, respectively. The probability of step-up at 1 year, 5 years and 10 years was 10, 18 and 25%, respectively. “Step-up patients” had higher rates of hospitalization, disease extent progression, extraintestinal manifestations and colonic stenosis and/or pseudo-polyps (p<0,05). Montreal E3 (HR:2,31;95% CI: 1,12-4,68), steroids at first flare (HR:3,45;95% CI: 1,26-7,29), lower 5-ASA maintenance dose (HR: 1,97;95%;CI:1,12-3,29), lower haemoglobin level (HR:1,45;95% CI: 1,11-3,24) and DUBLIN Score≥4 (HR:1,81;95% CI: 1,269-4,59) were factors associated with the step-up strategy, in multivariate analysis. Conclusion 5-ASA is an effective maintenance therapy; a satisfactory response was achieved in 72% of the patients after a mean time of follow-up of 10 years. Extensive colitis, higher DUBLIN Score, lower doses of 5-ASA, need for steroid and anaemia can predict a worse prognosis at the initial diagnosis and are associated with higher therapeutic requirements. These findings may be used in our clinical practice to identify patients that might benefit from an early step-up approach and minimize the morbidity associated with uncontrolled disease.