Bcl-6 protein expression, a marker of germinal center origin, has been associated with a favorable prognosis in diffuse large B-cell lymphoma (DLBCL). To determine the prognostic significance of this marker when rituximab (R) was added to cyclophosphamide, doxorubicin, vincristine, and prednisone (CHOP) chemotherapy, we prospectively studied Bcl-6 protein expression by immunohistochemical staining of 199 paraffin-embedded specimens from patients enrolled in the US Intergroup phase 3 trial comparing R-CHOP to CHOP with or without maintenance R. In Bcl-6(-) patients, failure-free survival (FFS) and overall survival (OS) were prolonged for those treated with R-CHOP alone compared to CHOP alone (2-year FFS 76% versus 9%, P < .001; 2-year OS 79% versus 17%, P < .001). In contrast, no differences in FFS and OS were detected between treatment arms for Bcl-6(+) cases. In the multivariate analysis, treatment arm (CHOP versus R-CHOP) was the major determinant of both FFS (P < .001) and OS (P < .001) for the Bcl-6(-) subset, whereas the International Prognostic Index risk group was the only significant predictor of outcome among Bcl-6(+) cases. Bcl-2 protein expression was not predictive of outcome in either group. In this study, we observed a reduction in treatment failures and death with the addition of R to CHOP in Bcl-6(-) DLBCL cases only. Our finding that Bcl-6(+) cases did not benefit from the addition of R to CHOP requires independent confirmation.