BACKGROUND The standard treatment for patients with diffuse large-B-cell lymphoma is cyclophosphamide, doxorubicin, vincristine, and prednisone (CHOP). Rituximab, a chimeric monoclonal antibody against the CD20 B-cell antigen, has therapeutic activity in diffuse large-B-cell lymphoma. We conducted a randomized trial to compare CHOP chemotherapy plus rituximab with CHOP alone in elderly patients with diffuse large-B-cell lymphoma. METHODS Previously untreated patients with diffuse large-B-cell lymphoma, 60 to 80 years old, were randomly assigned to receive either eight cycles of CHOP every three weeks (197 patients) or eight cycles of CHOP plus rituximab given on day 1 of each cycle (202 patients). RESULTS The rate of complete response was significantly higher in the group that received CHOP plus rituximab than in the group that received CHOP alone (76 percent vs. 63 percent, P=0.005). With a median follow-up of two years, event-free and overall survival times were significantly higher in the CHOP-plus-rituximab group (P<0.001 and P=0.007, respectively). The addition of rituximab to standard CHOP chemotherapy significantly reduced the risk of treatment failure and death (risk ratios, 0.58 [95 percent confidence interval, 0.44 to 0.77] and 0.64 [0.45 to 0.89], respectively). Clinically relevant toxicity was not significantly greater with CHOP plus rituximab. CONCLUSIONS The addition of rituximab to the CHOP regimen increases the complete-response rate and prolongs event-free and overall survival in elderly patients with diffuse large-B-cell lymphoma, without a clinically significant increase in toxicity.