Background The prognosis of young patients with diffuse large B-cell lymphoma at high risk (age-adjusted International Prognostic Index [aa-IPI] score 2 or 3) treated with R-CHOP (rituximab, cyclophosphamide, vincristine, doxorubicin, and prednisone) is poor. The aim of this study was to investigate the possible benefit of intensification with high-dose chemotherapy and autologous stem-cell transplantation as part of first-line treatment in these patients. Methods We did a multicentre, open-label, randomised, controlled, phase 3 trial with a 2 × 2 factorial design to compare, at two different R-CHOP dose levels, a full course of rituximab-dose-dense chemotherapy (no transplantation group) versus an abbreviated course of rituximab-dose-dense chemotherapy followed by consolidation with R-MAD (rituximab plus high-dose cytarabine plus mitoxantrone plus dexamethasone) and high-dose BEAM chemotherapy (carmustine, etoposide, cytarabine, and melphalan) plus autologous stem-cell transplantation (transplantation group) in young patients (18–65 years) with untreated high-risk diffuse large B-cell lymphoma (aa-IPI score 2–3). At enrolment, patients were stratified according to aa-IPI score and randomly assigned (1:1:1:1) to receive R-CHOP (intravenous rituximab 375 mg/m2, cyclophosphamide 750 mg/m2, doxorubicin 50 mg/m2, and vincristine 1·4 mg/m2 on day 1, plus oral prednisone 100 mg on days 1–5) delivered in a 14-day cycle (R-CHOP-14) for eight cycles; high-dose R-CHOP-14 (R-MegaCHOP-14; R-CHOP-14 except for cyclophosphamide 1200 mg/m2 and doxorubicin 70 mg/m2) for six cycles; R-CHOP-14 for four cycles followed by R-MAD (intravenous rituximab 375 mg/m2 on day 1 or 4 plus intravenous cytarabine 2000 mg/m2 and dexamethasone 4 mg/m2 every 12 h on days 1–3 plus intravenous mitoxantrone 8 mg/m2 on days 1–3) plus BEAM (intravenous carmustine 300 mg/m2 on day −7, intravenous cytarabine 200 mg/m2 twice a day on days −6 to −3, intravenous etoposide 100 mg/m2 twice a day on days −6 to −3, plus intravenous melphalan 140 mg/m2 on day −2) and autologous stem-cell transplantation (day 0); or R-MegaCHOP-14 for four cycles followed by R-MAD plus BEAM and autologous stem-cell transplantation. The primary endpoint was failure-free survival at 2 years in the intention-to-treat population. This study is registered with EudraCT (2005-002181-14; 2007-000275-42) and with ClinicalTrials.gov, number NCT00499018. Findings Between Jan 10, 2006, and Sept 8, 2010, 399 patients were randomly assigned to receive transplantation (n=199) or no transplantation (n=200); 203 patients were assigned to receive R-CHOP-14 and 196 were assigned to receive R-MegaCHOP-14. With a median follow-up of 72 months (IQR 57–88), 2-year failure-free survival was 71% (95% CI 64–77) in the transplantation group versus 62% (95% CI 55–68) in the no transplantation group (hazard ratio [HR] 0·65 [95% CI 0·47–0·91]; stratified log-rank test p=0·012). No difference in 5-year overall survival was observed between these groups (78% [95% CI 71–83] versus 77% [71–83]; HR 0·98 [0·65–1·48]; stratified log-rank test p=0·91). Grade 3 or worse haematological adverse events were reported in 183 (92%) of 199 patients in the transplantation group versus 135 (68%) of 200 patients in the no transplantation group. Grade 3 or worse non-haematological adverse events were reported in 90 (45%) versus 31 (16%); the most common grade 3 or worse non-haematological adverse event was gastrointestinal (49 [25%] vs 19 [10%]). Treatment-related deaths occurred in 13 (3%) patients; eight in the transplantation group and five in the no transplantation group. Interpretation Abbreviated rituximab-dose-dense chemotherapy plus R-MAD plus BEAM and autologous stem-cell transplantation reduced the risk of treatment failure compared with full course rituximab-dose-dense chemotherapy in young patients with diffuse large B-cell lymphoma at high risk. However, these results might not be clinically meaningful, since this improvement did not reflect an improvement in overall survival. These results do not support further consideration of the use of intensification of R-CHOP as an upfront strategy in patients with diffuse large B-cell lymphoma with poor prognosis. Funding Fondazione Italiana Linfomi.