The expression of the p73 gene and the methylation status was examined in 61 acute lymphoblastic leukemia (ALL) cell lines and lymphocytes from seven healthy individuals. p73 mRNA was not expressed in 19 (31.1%) of 61 ALL cell lines, including 11 (31.4%) of 35 B-precursor ALL cell lines, 2 (16.7%) of 12 B-ALL/Burkitt lymphoma (BL) cell lines (totally 27.7% of B-lineage cell lines), 6 (42.9%) of 14 T-ALL cell lines, and expressed in all of normal lymphocytes, by reverse transcriptase-polymerase chain reaction (RT-PCR). Restriction-enzyme related PCR (REP) and methylation-specific PCR (MSP) revealed that the cell lines lacking p73 mRNA expression were hypermethylated. In contrast, normal lymphocytes and most cell lines that expressed detectable p73 mRNA were not hypermethylated with the exception of five cell lines. Furthermore, bisulfite genomic sequencing confirmed the results obtained by REP and MSP. Our results suggest that p73 inactivation may be involved in the pathogenesis of both T- and B-ALLs, and that hypermethylation is the predominant mechanism of inactivation of the p73 gene in ALL.