High-frequent silencing of hematopoietic cell-specific protein-tyrosine phosphatase SHP1 gene by promoter methylation was detected in various kinds of leukemias and lymphomas, as well as in many hematopoietic cell lines, which is supported by our previous observation of strong decrease of SHP1 mRNA and protein. The promoter methylation of the SHP1 gene was clearly correlated with the clinical stage. Loss of heterozygosity with microsatellite markers near the SHP1 gene was shown in 79% of informative acute lymphoblastic leukemia cases. These results suggest that functional loss of SHP1 is associated with the pathogenesis of leukemias/lymphomas.