Summary Background The role of adjuvant chemotherapy for patients with rectal cancer is controversial, especially when used after preoperative chemoradiotherapy. Fluoropyrimidine-based adjuvant chemotherapy, including fluorouracil and leucovorin, has been widely used; however, the addition of oxaliplatin to fluorouracil and leucovorin (FOLFOX), a standard adjuvant regimen for colon cancer, has not been tested in rectal cancer. We aimed to compare the efficacy and safety of adjuvant fluorouracil and leucovorin with that of FOLFOX in patients with locally advanced rectal cancer after preoperative chemoradiotherapy. Methods In this open-label, multicentre, phase 2, randomised trial, patients with postoperative pathological stage II (ypT3–4N0) or III (ypTany N1–2) rectal cancer after preoperative fluoropyrimidine-based chemoradiotherapy and total mesorectal excision were recruited and randomly assigned (1:1) via a web-based software platform to receive adjuvant chemotherapy with either four cycles of fluorouracil and leucovorin (fluorouracil 380 mg/m2 and leucovorin 20 mg/m2 on days 1–5, every 4 weeks) or eight cycles of FOLFOX (oxaliplatin 85 mg/m2 , leucovorin 200 mg/m2 , and fluorouracil bolus 400 mg/m2 on day 1, and fluorouracil infusion 2400 mg/m2 for 46 h, every 2 weeks). Stratification factors were pathological stage (II vs III) and centre. Neither patients nor investigators were masked to group assignment. The primary endpoint was 3-year disease-free survival, analysed by intention to treat. This study is fully enrolled, is in long-term follow-up, and is registered with ClinicalTrials.gov , number NCT00807911. Findings Between Nov 19, 2008, and June 12, 2012, 321 patients were randomly assigned to fluorouracil and leucovorin (n=161) and FOLFOX (n=160). 141 (95%) of 149 patients in the fluorouracil plus leucovorin group and 141 (97%) of 146 in the FOLFOX group completed all planned cycles of adjuvant treatment. Median follow-up was 38·2 months (IQR 26·4–50·6). 3-year disease-free survival was 71·6% (95% CI 64·6–78·6) in the FOLFOX group and 62·9% (55·4–70·4) in the fluorouracil plus leucovorin group (hazard ratio 0·657, 95% CI 0·434–0·994; p=0·047). Any grade neutropenia, thrombocytopenia, fatigue, nausea, and sensory neuropathy were significantly more common in the FOLFOX group than in the fluorouracil plus leucovorin group; however, we noted no significant difference in the frequency of these events at grade 3 or 4. The most common grade 3 or worse adverse events were neutropenia (38 26% of 149 patients in the fluorouracil plus leucovorin group vs 52 36% of 146 patients in the FOLFOX group), leucopenia (eight 5% vs 12 8%), febrile neutropenia (four 3% vs one <1%), diarrhoea (four 3% vs two 1%), and nausea (one <1% vs two 1%). Interpretation Adjuvant FOLFOX improves disease-free survival compared with fluorouracil plus leucovorin in patients with locally advanced rectal cancer after preoperative chemoradiotherapy and total mesorectal excision, and warrants further investigation. Funding Korea Healthcare Technology R&D Project (South Korean Ministry of Health and Welfare).