Microsatellite instability (MSI) is a recently described type of genetic alteration resulting from defects in the DNA mismatch repair genes that appears to play an integral role in neoplastic transformation. MSI has been described in a wide variety of malignancies; however, data regarding the role of MSI in the pathogenesis of non-Hodgkin's lymphoma (NHL) are limited. MSI appears to be important in some T-cell lymphomas, including ALCL arising in immunocompromised patients. In addition, MSI has recently been identified in CD30+ cutaneous lymphoproliferative processes and lymphoblastic lymphoma. In this study, we have analyzed five well-characterized cases of systemic T-cell ALCL arising in immunocompetent patients for the presence of MSI. Genomic DNA isolated from paired normal and tumor tissue was analyzed at seven microsatellite loci by polymerase chain reaction. We were unable to identify MSI or loss of heterozygosity (LOH) in our cases, suggesting that abnormalities in the DNA mismatch repair system do not play a major role in the pathogenesis of most systemic ALCL. Our data provide additional molecular evidence that the various subgroups of lymphoma with ALCL morphology are biologically distinct processes.