We describe here the first well-characterized case of "composite" lymphoma of the spleen in which the two components were a low-grade and a high-grade B-cell non-Hodgkin's lymphomas. The patient was an elderly man with prominent splenomegaly and multiple hypoechogenic lesions of the spleen. A splenectomy was performed, and the macroscopic and histological findings showed the simultaneous presence of a "low-grade" B-cell lymphoma, lymphoplasmacytoid (immunocytoma) and a "high-grade" B-cell lymphoma (immunoblastic), which were spatially separated. The two lesions expressed the same immunoglobulin light chain (lambda), but the Southern blot analysis showed different patterns of immunoglobulin heavy chain (IgH) clonal rearrangement. PCR analysis followed by direct sequencing of the IgH-amplified rearrangement products provided molecular-genetic evidence that the two components of the composite lymphoma had the same clonal origin. Since both EBV LMP-1 and p53 were negative by immunohistochemistry, it is unlikely that EBV and p53 were involved in the neoplastic progression in this case. PCR analysis and direct sequencing of IgH-amplified rearrangement products are useful tools to investigate clonality in cases in which Southern blot analysis cannot be performed or does not provide conclusive findings.