Summary Background Rituximab plus chemotherapy, most often CHOP (cyclophosphamide, doxorubicin, vincristine, and prednisone), is the first-line standard of care for patients with advanced indolent lymphoma, and for elderly patients with mantle-cell lymphoma. Bendamustine plus rituximab is effective for relapsed or refractory disease. We compared bendamustine plus rituximab with CHOP plus rituximab (R-CHOP) as first-line treatment for patients with indolent and mantle-cell lymphomas. Methods We did a prospective, multicentre, randomised, open-label, non-inferiority trial at 81 centres in Germany between Sept 1, 2003, and Aug 31, 2008. Patients aged 18 years or older with a WHO performance status of 2 or less were eligible if they had newly diagnosed stage III or IV indolent or mantle-cell lymphoma. Patients were stratified by histological lymphoma subtype, then randomly assigned according to a prespecified randomisation list to receive either intravenous bendamustine (90 mg/m2 on days 1 and 2 of a 4-week cycle) or CHOP (cycles every 3 weeks of cyclophosphamide 750 mg/m2 , doxorubicin 50 mg/m2 , and vincristine 1·4 mg/m2 on day 1, and prednisone 100 mg/day for 5 days) for a maximum of six cycles. Patients in both groups received rituximab 375 mg/m2 on day 1 of each cycle. Patients and treating physicians were not masked to treatment allocation. The primary endpoint was progression-free survival, with a non-inferiority margin of 10%. Analysis was per protocol. This study is registered with ClinicalTrials.gov , number NCT00991211 , and the Federal Institute for Drugs and Medical Devices of Germany, BfArM 4021335. Findings 274 patients were assigned to bendamustine plus rituximab (261 assessed) and 275 to R-CHOP (253 assessed). At median follow-up of 45 months (IQR 25–57), median progression-free survival was significantly longer in the bendamustine plus rituximab group than in the R-CHOP group (69·5 months 26·1 to not yet reached vs 31·2 months 15·2–65·7; hazard ratio 0·58, 95% CI 0·44–0·74; p<0·0001). Bendamustine plus rituximab was better tolerated than R-CHOP, with lower rates of alopecia (0 patients vs 245 (100%) of 245 patients who recieved ≥3 cycles; p<0·0001), haematological toxicity (77 30% vs 173 68%; p<0·0001), infections (96 37% vs 127 50%); p=0·0025), peripheral neuropathy (18 7% vs 73 29%; p<0·0001), and stomatitis (16 6% vs 47 19%; p<0·0001). Erythematous skin reactions were more common in patients in the bendamustine plus rituximab group than in those in the R-CHOP group (42 16% vs 23 9%; p=0·024). Interpretation In patients with previously untreated indolent lymphoma, bendamustine plus rituximab can be considered as a preferred first-line treatment approach to R-CHOP because of increased progression-free survival and fewer toxic effects. Funding Roche Pharma AG, Ribosepharm/Mundipharma GmbH.