Background and Aims: Disease extent in ulcerative colitis [UC] is dynamic and can progress over time. Little is known about risk factors for UC extension in the era of biologics. We investigated the risk of UC extension and subsequent risk of surgery in a Danish population-based cohort. Methods: All incident UC cases in a strictly defined Copenhagen area between 2003 and 2004 were followed prospectively through 2011. Disease extension was defined as patients with limited UC [E1 or E2] at diagnosis having progressed from the initial extent by colonoscopy or surgery to E2 or extensive colitis [E3]. Associations between progression or colectomy and multiple covariates were analysed by Cox regression analysis. Results: Of 300 UC patients, 220 [73%] had E1 or E2 at diagnosis. During follow-up, 50 [23%] patients with E1/E2 progressed to E3, and 22 [10%] with E1 progressed to E2. Disease extent at diagnosis was the sole predictor of extension to E3. A total of 18 [8%] patients with E1/E2 at diagnosis had a colectomy. Progression from E1/E2 to E3, female gender and a history of smoking were risk factors for colectomy. Conclusion: After 7 years of follow-up, 33% of patients with limited UC experienced disease extension. Only extent at diagnosis was a clinical predictor for disease extension. The risk of colectomy was increased in former smokers and patients who progressed to extensive colitis. This highlights the need to prevent disease progression in patients with limited UC, and to identify new histological or molecular markers that might help stratify risks for disease progression.