Bone morphogenetic proteins (BMP), belonging to the transforming growth factor-beta superfamily, are multifunctional regulators of cell proliferation, differentiation and apoptosis in various types of malignant cells. In this study, we investigated BMP-6 promoter methylation in patients with various types of leukemias. The BMP-6 methylation was found preferentially in adult T-cell leukemia (ATL) (49 of 60, 82%) compared with other types of leukemias studied including acute myeloid leukemia (3 of 67, 5%), acute lymphoblastic leukemia (6 of 38, 16%) and chronic lymphocytic leukemia (1 of 21, 5%). Among subtypes of ATL, the BMP-6 gene was more frequently methylated in aggressive ATL forms of acute (96%) and lymphoma (94%) types than less malignant chronic ATL (44%) and smoldering ATL (20%). We also analyzed the methylation status of peripheral blood mononuclear cells from healthy donors and nonmalignant lymph nodes with reactive lymphadenopathy, none of which showed detectable BMP-6 methylation in this study. The BMP-6 methyaltion was correlated with decreased mRNA transcript and protein expression. Expression of BMP-6 was restored by the demethylating agent 5-aza-2'-deoxycytidine, suggesting that methylation was associated with the transcriptional silencing. Serial analysis demonstrated an increasing methylation of CpG sites in the BMP-6 promoter and the resultant suppression of BMP-6 expression as ATL progressed. These findings suggested that BMP-6 promoter methylation is likely to be a common epigenetic event at later stages of ATL and that the methylation profiles may be useful for the staging of ATL as well as for evaluation of the individual risk of developing the disease.