Genetic instability has been recently related to point mutations on genes involved in DNA repair pathway of errors produced during replication. These molecular alterations have been described in hereditary and sporadic colon carcinomas and related tumors. To examine genetic instability on lympho- and myeloproliferative processes, we analyzed the behaviour of 10 microsatellite markers and one VNTR on different chromosomes in 10 patients with non-Hodgkin lymphomas (NHL), one patient with acute lymphoblastic leukemia (ALL) and 10 patients with acute myeloid leukemia (AML). Mobility shifts were found in three of those cases. One of them showed genetic instability for several markers--microsatellites and VNTR- and the other two showed differences for only one marker. As a correlation between point mutations in MSH2 gene and the presence of genetic instability in hereditary non-polyposis colon cancer (HNPCC) and related tumors has been found, we analyzed the sequence of a conversed region of this gene in the cases showing this phenomenon. We only found a polymorphism, previously described, in 669 codon from cDNA.