Approximately 5% of diffuse large B-cell lymphomas (DLBCLs) are double-hit lymphomas (DHLs) with translocations of both MYC and BCL2. DHLs are characterized by poor outcome. We tested whether DLBCLs with high expression of MYC protein and BCL2 protein share the clinical features and poor prognosis of DHLs. Paraffin-embedded lymphoma samples from 193 patients with de novo DLBCL who were uniformly treated with rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisone (R-CHOP) were studied using immunohistochemistry for MYC, BCL2, CD10, BCL6, and MUM1/interferon regulatory factor 4, and fluorescent in situ hybridization (FISH) for MYC and BCL2. FISH analysis identified DHL in 6% of patients, who showed the expected poor overall survival (OS; P=. 002). On the basis of immunohistochemical MYC and BCL2 expression, a double-hit score (DHS) was assigned to all patients with DLBCL. The DHS-2 group, defined by high expression of both MYC and BCL2 protein, comprised 29% of the patients. DHS 2 was significantly associated with lower complete response rate (P=. 004), shorter OS (P<. 001), and shorter progression-free survival (PFS; P<. 001). The highly significant correlation with OS and PFS was maintained in multivariate models that controlled for the International Prognostic Index and the cell-of-origin subtype (OS, P<. 001; PFS, P<. 001). DHS was validated in an independent cohort of 116 patients who were treated with R-CHOP. The immunohistochemical DHS defined a large subset of DLBCLs with double-hit biology and was strongly associated with poor outcome in patients treated with R-CHOP.