We describe a Warthin's tumor which was involved by malignant lymphoma. The lymphoma was classified in the Working Formulation as follicular and diffuse, mixed small cleaved and large cell type. Frozen section immunohistochemical studies revealed an abnormal immunophenotype: immunoglobulin-negative and B-lineage. Gene rearrangement analysis confirmed the diagnosis by demonstrating rearrangements of both the immunoglobulin heavy and kappa light chain genes. The bcl-2 gene was also rearranged, consistent with the presence of the t(14;18) (q32;q21) translocation which is typically seen in follicular lymphomas. The T-cell receptor beta chain gene retained the germline configuration. The results in this case highlight an advantage of molecular techniques as compared with immunophenotypic analysis: gene expression is not required to demonstrate clonality.