Tissue eosinophilia is commonly seen in Hodgkin's disease and non-Hodgkin's lymphomas of T-cell lineage. In contrast, eosinophilia is infrequent in non-Hodgkin's lymphomas of B-cell origin. We describe five-B-cell lymphomas with exuberant tissue eosinophils. According to the Working Formulation, three were classified as large-cell immunoblastic, one as small lymphocytic lymphoma/chronic lymphocytic leukemia, and one as low-grade, not further subclassified, with features of monocytoid B-cell lymphoma. Immunophenotypic studies in each case revealed B-cell lineage; neoplastic cells expressed monotypic immunoglobulin light chain (four of five cases) or pan-B-cell antigens (five of five cases) and were negative for T-cell antigens. Southern blot hybridization in one case revealed immunoglobulin gene rearrangements, further confirming B-cell lineage. Eosinophilopoiesis is stimulated by interleukin 5 (IL-5), and Epstein-Barr virus (EBV) has been shown to upregulate IL-5 production. Therefore, both EBV infection and IL-5 expression were investigated as possibly pathogenetic mechanisms for the eosinophilia. However, both in situ hybridization studies for EBV mRNA and IL-5 mRNA were negative in the neoplastic cells. In one tumor, IL-5 was abundant in the cytoplasm of the eosinophils, a pattern similar to that seen in five cases of Hodgkin's disease studied with the same technique. Although rare, marked tissue eosinophilia may be associated with B-cell non-Hodgkin's lymphomas. Immunophenotypic or molecular genetic analyses are needed to make the correct diagnosis.