We investigated mismatch repair (MMR) gene expression in 31 lymphoid tissue specimens and bone marrow aspirates with malignant lymphoproliferative disorders of B-cell origin (25 cases of lymphoma and six cases of plasma cell myeloma). A multiplex RT-PCR assay was employed to assess the relative expression of the hMSH2, hMLH1 and hPMS1 genes, as compared to beta-actin, which was used as an internal control of gene expression. MSH2 was further evaluated at the protein level by immunohistochemistry. The findings were compared to those of a control group of lymphoid tissue specimens without evidence of malignancy (n = 6). Changes in MMR gene expression were observed in 10 out of 31 cases of the study group (32%). All three MMR gene transcripts were low in two out of six plasma cell myelomas, which had extensive bone marrow infiltration by neoplastic cells. The hMSH2 transcript was present in all cases of lymphoma, while the expression of hMLH1 and hPMS1 was significantly low in some large B-cell lymphomas (four and five out of 14 cases, respectively) and in mantle cell lymphomas of the blastoid type (two out of two cases). No MMR gene aberrations were found in seven cases of B-cell lymphocytic leukemia and two cases of mantle cell lymphoma of centrocyte-like type. These findings demonstrate that the expression rates of the hMSH2, hMLH1 and hPMS1 genes differ among various types of B-cell lymphoproliferative disorders, and suggest that MMR gene expression may be related to the natural history of these neoplasms. This study identified a higher incidence of MMR gene aberrations in lymphoma types characterized by aggressive biologic behavior, as compared to neoplasms with a more indolent course.