CONTEXT: No known published study has focused on a plant-based diet (PBD) in the treatment of ulcerative colitis (UC). OBJECTIVE: To investigate relapse prevention in UC after consumption of a PBD during educational hospitalization in Japan. DESIGN: Prospective study of patients with mild UC or UC in remission who did not need immediate treatment. A PBD and dietary guidance were provided during a two-week hospitalization. MAIN OUTCOME MEASURES: The primary end point was relapse (a flare-up that required more aggressive treatment) during the follow-up period. Kaplan-Meier analysis was used to calculate the cumulative relapse rate. Secondary end points were immediate improvement in symptoms or laboratory data during hospitalization and a chronologic change in the PBD score, which evaluated adherence to the PBD. RESULTS: Sixty cases were studied: 29 initial episode cases and 31 relapse cases. Of these, 31 involved proctitis; 7, left-sided colitis; and 22, extensive colitis. Thirty-seven patients were receiving medication; 23 were not. The median age was 34 years; median follow-up was 3 years 6 months. Eight cases relapsed during follow-up. The cumulative relapse rates at 1, 2, 3, 4, and 5 years of follow-up were 2%, 4%, 7%, 19%, and 19%, respectively. Most patients (77%) experienced some improvement such as disappearance or decrease of bloody stool during hospitalization. The short- and long-term PBD scores after the hospitalization were higher than baseline PBD scores. CONCLUSION: Relapse rates after educational hospitalization providing a PBD were far lower than those reported with medication. Educational hospitalization is effective at inducing habitual dietary changes.