The simultaneous diagnosis of marginal zone lymphoma of mucosa-associated lymphoid tissue (MALT) and chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL) is rare. This study reports a patient with composite synchronous biclonal occurrence of MALT lymphoma of the lung and CLL/SLL. The morphology of the lung and peripheral blood showed features of MALT lymphoma and CLL, respectively. The cytogenetic evaluation of the lung specimen revealed a t(1;14) (p22;q32), a frequent genetic abnormality in MALT lymphoma. Flow cytometry analysis of the lung tissue showed features of MALT lymphoma and CLL/SLL with different light chain restriction, whereas the blood showed phenotypic evidence of CLL/SLL. Fluorescence in situ hybridization study of the blood showed a deletion of 13q14 and 17p13. Immunoglobulin heavy chain (IgH) gene rearrangement study of the lung tissue and blood showed a monoclonal IgH gene rearrangement with distinct light chain restriction, suggesting that the immunophenotypically different cell populations originated from separate clones.