In a patient with small lymphocytic proliferation (SLP) involving the right middle and right lower lobes of the lung, immunophenotypic studies showed that the neoplastic lymphoid cells in the right middle lobe expressed κ light chains, whereas those in the right lower lobe expressed λ light chains on their surface. Gene rearrangement studies with Southern-blot hybridization confirmed the disparate surface immunoglobulin expression, and showed that the SLPs in the two lobes were derived from separale clones. The findings indicate that even morphologically identical lymphomas in the same organ may be immunophenotypically and genotypically heterogeneous. Our findings demonstrate that immunologic and DNA gene rearrangement analyses may complement each other in the study of lymphomas. This case is unique in that it is the first reported case of the concurrent presence of two immunologically distinct clonal populations in an extranodal site.