Double-hit (DH) lymphomas with MYC and either BCL2 (DH-BCL2/MYC) or BCL6 (DH-BCL6/MYC) rearrangements are considered very aggressive, many of which are now included in the category B-cell lymphoma, unclassifiable, with features intermediate between diffuse large B-cell lymphoma (DLBCL) and Burkitt lymphoma (BL) (DLBCL/BL). However, data describing the DH cases are largely based on DH-BCL2/MYC cases. To better characterize DH-BCL6/MYC cases, the clinical, morphologic, phenotypic, and cytogenetic features of 6 cases from University of Pittsburgh Medical Center and 17 cases from the Mitelman database were reviewed. In the University of Pittsburgh Medical Center cases, the median age was 83 years (range, 51 to 89 y) with 5/6 DLBCL/BL cases and 1 large B-cell lymphoma, not otherwise specified. Five of 6 had a germinal center phenotype, 1/6 was BCL2(+), and the median Ki-67 score was 98% (35% to 100%). The Mitelman DH-BCL6/MYC cases included 13 aggressive B-cell lymphomas (diagnosed as DLBCL-5, BL-5, BL-like lymphomas-2, and primary effusion lymphoma-1) and 4 other lymphoid/plasmacytic neoplasms. The median cytogenetic complexity score was 2.5 (range, 0 to 14) in 14 evaluable mature aggressive lymphomas with an immunoglobulin gene partner for MYC in 9/14 and for BCL6 in 7/14 cases. Ten of 13 cases involved extranodal extramedullary sites at presentation, and the median survival for the 10 patients with large cell neoplasms or BL and with available follow-up data was 9 months. Thus, DH-BCL6/MYC lymphomas are aggressive, frequently involve extranodal sites, and are often DLBCL/BL with a germinal center phenotype. Unlike DH-BCL2/MYC lymphomas, however, they are more likely to be CD10(-) but IRF4/MUM-1(+) (P=0.03) and, more like BL, only infrequently express BCL2 (P<0.001), and are cytogenetically less complex (P<0.04).