As mice carrying mutations of the DNA mismatch repair genes MSH2 and MSH6 often develop lymphoid neoplasms, we addressed the prevalence of the replication error (RER(+)) phenotype, a manifestation of an underlying defect of DNA mismatch repair genes, in human lymphoid tumors. We compared microsatellite instability (MSI) at 10 loci in 37 lymphoid tumors, including 16 acute lymphoid leukemias (ALL) and 21 non-Hodgkin's lymphomas (NHL), and in 29 acute myeloid leukemias (AML). Significant differences in MSI prevalence between AMLs and ALLs emerged, and MSI occurrence was more frequent in the NHLs versus AMLs. Indeed, only 3 of 29 (10%) AMLs exhibited MSI, thus confirming its paucity in myeloid tumors, while 10 of 37 (27%) lymphoid tumors, 6 ALLs and 4 NHLs, disclosed an RER(+) phenotype. In 1 ALL patient, the same molecular alterations were observed in correspondence with a relapse, but were not detected during remission over a 14-month follow-up; in another ALL patient, findings correlated with impending clinical relapse. These results suggest that the study of MSI in lymphoid tumors might provide a useful molecular tool to monitor disease progression in a subset of ALLs. To correlate MSI with other known genetic abnormalities, we investigated the status of the proto-oncogene, bcl-2, in the lymphoma patients and found that 4 of 4 NHL patients with MSI carried bcl-2 rearrangements, thus linking genomic instability to enhanced cell survival in NHL; moreover, no p53 mutations were found in these patients. Finally, we addressed the putative cause of MSI in hematopoietic tumors by searching for both mutations and deletions affecting DNA repair genes. A limited genetic analysis did not show any tumor-specific mutation in MLH1 exons 9 and 16 and in MSH2 exons 5 and 13. However, loss of heterozygosity (LOH) of markers closely linked to mismatch repair genes MLH1, MSH2, and PMS2 was demonstrated in 4 of 6 ALLs and 1 of 3 AMLs with MSI. These observations indicate that chromosomal deletions might represent a mechanism of inactivation of DNA repair genes in acute leukemia.