Background: The role of rituximab in combination with different CHOP (cyclophosphamide, doxorubicin, vincristine, and prednisone)-like chemotherapy regimens in young patients with good-prognosis diffuse large-B-cell lymphoma remains to be defined. We aimed to compare CHOP-like chemotherapy and rituximab with CHOP-like chemotherapy alone in these patients. Methods: 824 patients who were from 18 countries; aged 18-60 years; and who had no risk factors or one risk factor according to age-adjusted International Prognostic Index (IPI), stage II-IV disease, or stage I disease with bulk were enrolled. These patients were randomly assigned to six cycles of CHOP-like chemotherapy and rituximab (n=413) or to six cycles of CHOP-like chemotherapy alone (n=411). Bulky and extranodal sites received additional radiotherapy. The primary endpoint was event-free survival; secondary endpoints were response, progression under therapy, progression-free survival, overall survival, and frequency of toxic effects. Analyses were done by intention to treat and per protocol. This trial is registered at http://www.clinicaltrials.gov, NCT 00064116. Findings: After a median follow-up of 34 months (range 0·03-61), patients assigned chemotherapy and rituximab had increased 3-year event-free survival compared with those assigned chemotherapy alone (79% [95% CI 75-83] vs 59% [54-64]; difference between groups 20% [13-27], log-rank p<0·0001), and had increased 3-year overall survival (93% [90-95] vs 84% [80-88]; difference between groups 9% [3-13], log-rank p=0·0001). Event-free survival was affected by treatment group, presence of bulky disease, and age-adjusted IPI: after chemotherapy and rituximab, a favourable subgroup (ie, IPI=0, no bulk) could be defined from a less-favourable subgroup (ie, IPI=1 or bulk, or both). Groups did not differ in the frequency of adverse events. Interpretation: Rituximab added to six cycles of CHOP is an effective treatment for young patients with good-prognosis diffuse large-B-cell lymphoma. The definition of two prognostic subgroups allows for a more refined therapeutic approach for these patients.