Background and Aims: The GEMINI long-term safety [LTS] study is a continuing phase 3 trial investigating the safety and efficacy of vedolizumab, an alpha(4)beta(7) integrin antagonist for ulcerative colitis [UC] and Crohn's disease. We provide an interim analysis of efficacy in patients with UC. Methods: Patients from the C13004 and GEMINI 1 studies and a cohort of vedolizumab-naive patients received open-label vedolizumab every 4 weeks. Interim data were collected from May 22, 2009 to June 27, 2013. Clinical response and remission, evaluated using partial Mayo scores, and health-related quality of life [HRQL] were assessed for up to 152 weeks of cumulative treatment in the efficacy population. Results: As of June 27, 2013, 63% of the efficacy population [n = 532/845] were continuing treatment. Among patients who responded to vedolizumab induction and had data available, 88% [n = 120/136] were in remission after 104 weeks of exposure (96% [n = 70/73] after 152 weeks). Among patients who withdrew from every-8-week vedolizumab maintenance in GEMINI 1 [n = 32] before week 52, increased dosing to every 4 weeks in GEMINI LTS resulted in response and remission rates of 41% and 28%, respectively, after 52 weeks, an increase from 19% and 6%, respectively, from before the dose increase. Similar benefits were demonstrated regardless of prior tumour necrosis factor-antagonist exposure. Durable benefits on HRQL were also observed. Conclusions: Patients with UC experienced clinical and HRQL improvements with continued vedolizumab treatment. Increased dosing frequency to every 4 weeks was beneficial in patients who had loss of response to 8-weekly dosing.