p300 is a widely expressed transcriptional coactivator, and is involved in DNA repair, cell growth, differentiation, and apoptosis. Recent data suggest that it may function as a tumor suppressor. We investigated the frequency of aberrant methylation of p300 promoter in seven leukemic cell lines, as well as in the diagnostic samples of 46 patients with acute myelocytic leukemia (11 with M1, 22 with M2, 3 with M4, and 1 with M5), 24 patients with acute promyelocytic leukemia, and 22 patients with acute lymphoblastic leukemia (3 with T-cell, 1 with pre-B, 2 with Burkitt, 2 with early B-precursors, and 14 with common). None of the cell lines and patient samples showed p300 gene methylation. Therefore, hypermethylation of p300 is not an important mechanism in leukemogenesis.