Approximately 60% of patients with diffuse large B-cell non-Hodgkin lymphoma (DLBCL) are curable with rituximab plus cyclophosphamide, doxorubicin, vincristine, and prednisone (CHOP) chemoimmunotherapy. Epratuzumab (E) is an unlabeled anti-CD22 monoclonal antibody with efficacy in relapsed DLBCL. This phase 2 trial tested the safety and efficacy of combining E with R-CHOP (ER-CHOP) in untreated DLBCL. A secondary aim was to assess the efficacy of interim positron emission tomography (PET) to predict outcome in DLBCL. Standard R-CHOP with the addition of E 360 mg/m(2) intravenously was administered for 6 cycles. A total of 107 patients were enrolled in the study. Toxicity was similar to standard R-CHOP. Overall response rate in the 81 eligible patients was 96% (74% CR/CRu) by computed tomography scan and 88% by PET. By intention to treat analysis, at a median follow-up of 43 months, the event-free survival (EFS) and overall survival (OS) at 3 years in all 107 patients were 70% and 80%, respectively. Interim PET was not associated with EFS or OS. Comparison with a cohort of 215 patients who were treated with R-CHOP showed an improved EFS in the ER-CHOP patients. ER-CHOP is well tolerated and results appear promising as a combination therapy. This study was registered at www.clinicaltrials.gov as #NCT00301821.