Aims: To characterize the clinicopathological and genetic features of pleomorphic mantle cell lymphoma (PMCL), which morphologically mimics diffuse large B cell lymphoma (DLBCL). Methods and results: We screened systematically 500 B cell lymphomas morphologically compatible with DLBCL using an immunohistochemical algorithm of three markers (CD5, cyclin D1 and SOX11). Ten cases of PMCL were identified for further study and, surprisingly, four (40%) of them were cyclin D1-negative. These 10 patients were mainly elderly males with advanced disease, and their median survival was only 11 months. All cyclin D1-positive PMCLs tested showed an IGH-CCND1 translocation, whereas one of the four cyclin D1-negative PMCLs had a translocation involving CCND2 and a high CCND2 mRNA level (P < 0.000001). The genomewide copy number profiles of both cyclin D1-positive and cyclin D1negative PMCLs were similar to those of classical mantle cell lymphoma (MCL) reported previously, confirming the diagnosis. Secondary genetic alterations involved in oncogenic pathways of MCL were observed more frequently in these PMCLs, possibly decreasing the dependence on the driving CCND1 translocation and accounting for the common cyclin D1 negativity. Copy number gains of PIK3CA and CCDC50 were detected in all cyclin D1-negative PMCLs but in only 40% of the cyclin D1-positive PMCLs. These additional oncogenic signals may compensate for the common absence of CCND2 translocation in cyclin D1-negative PMCL. Conclusion: We demonstrate for the first time that cyclin D1 negativity is surprisingly common in PMCL morphologically mimicking DLBCL, and the use of a simple immunohistochemical algorithm can prevent misclassification and inappropriate treatment.