We report the case of a 71-year-old woman presenting with composite lymphoma (CL) composed of a follicular lymphoma and a B-cell chronic lymphocytic leukemia. CL is a rare lymphoproliferative disorder, characterized by two distinct morphological and immunophenotypical patterns in the same anatomical site, most frequently of biclonal origin. This entity must be distinguished from transformation of low-grade lymphoma into high-grade lymphoma and from lymphoma with differentiation such as follicular lymphoma with marginal differentiation. In this context, molecular analysis including immunoglobulin rearrangement, sequencing and FISH analyses is determinant and can be improved by tissue microdissection. Routinely, CL must not be misdiagnosed because of its prognosis and treatment implication.