['To characterize the frequency and clinicopathological features of cyclin D1-positive diffuse large B-cell lymphoma (DLBCL) and the usefulness of SOX11 in the differential diagnosis from mantle cell lymphoma (MCL).', 'We retrospectively stained 206 consecutive DLBCLs for cyclin D1, and identified three (1.5%) positive cases, comprising two in the elderly with necrosis, and a third with a starry-sky pattern. All three cases shared the same non-germinal centre B-cell (non-GCB) phenotype [CD5-/CD10-/bcl-6+/MUM1+/SOX11-], Epstein-Barr virus (EBV) negativity, and absence of CCND1 aberrations by fluorescence in-situ hybridization. The third case showed both BCL6 and MYC rearrangements: a double-hit lymphoma. In the same period there were 22 MCLs, all expressing cyclin D1, with 89% cases expressing SOX11, a frequency that is statistically different from cyclin D1-positive DLBCL. Notably, we identified a pleomorphic MCL initially misdiagnosed as DLBCL. A separate cohort of 98 DLBCL cases was negative for SOX11, with only one case expressing cyclin D1 with a GCB phenotype (CD10+/bcl-6+/MUM1-). The two patients with tumour necrosis rapidly died of disease. The other two were in complete remission after immunochemotherapy.'] Is sOX11 useful in differentiating cyclin D1-positive diffuse large B-cell lymphoma from mantle cell lymphoma?