Both p16 and p15, encoded by genes located on chromosome 9p21, are inhibitors of cyclin-dependent kinases 4/6 (CDK4/6) and upstream regulators of RB function, and set up the RB/p16 tumor suppressive pathway, which is abrogated frequently in human neoplasms, either through inactivation of the RB or p16 tumor-suppressor protein, or alteration of the cyclin D1 or CDK4 oncoproteins. In hematological malignancies, deletion of p16/p15 locus has been shown to be highly specific to lymphoid malignancies, and more particularly to T-cell acute lymphoblastic leukemia (T-ALL). However, in the other subsets of ALL, deletions of p16 and p15 are relatively rare events. To investigate whether these genes are inactivated by methylation of the 5' CpG islands, we examined 35 leukemia cell lines and 29 childhood acute myeloid leukemia (AML) patients by Southern blot, polymerase chain reaction (PCR) and Western blot analyses. We found methylation of p16 in 12 (50%) of 24 ALL cell lines, 5 (50%) of 10 AML cell lines without homozygous deletion of p16, and 11 (38%) of 29 AML patients. Those leukemia cell lines subjected to p16 methylation were found to have lost p16 protein expression. The p15 gene was methylated in 10 (34%) of 29 ALL cell lines, 6 (60%) of 10 AML cell lines without homozygous deletion of p15, and 15 (52%) of 29 AML patients. These results revealed the frequent methylation of p16 and p15 genes in B-ALL and AML despite a low frequency of p16 and p15 deletions and mutations in these leukemias. In the study for expression of RB protein, we found no expression of RB in 4 of 16 leukemia cell lines. Inactivation of the p16 gene was found in all the cell lines with expression of RB. Neither amplification nor rearrangement of cyclin D1 gene was found in any cell lines. These results suggest that inactivation of p16 and p15 genes is one of the most common genetic events in acute leukemia, and plays an important role for the RB/p16 pathway in the pathogenesis of acute leukemia.