Composite lymphomas are rare and involve the concurrent evolution of 2 distinct lymphoma types within a single organ or tissue. This study describes 2 cases of composite mantle cell lymphoma (MCL) and diffuse large B-cell lymphoma (DLBCL), which has not previously been reported. Each case demonstrated distinct populations of CD20 positive small and large atypical B cells. In both cases, only the small lymphocytes were positive for CD5 and cyclin D1, and fluorescence in situ hybridization (FISH) showed a t(11;14) translocation in the small lymphocytes but not in the large cells. Molecular studies for B-cell clonality showed a possible clonal relationship between the 2 components in one case but not the other. This study describes in detail the morphology, immunophenotype, FISH, and molecular analysis of both components in each case. To the authors' knowledge, this represents the first report of juxtaposition of MCL with DLBCL that does not represent transformation of the mantle cell component.