Isolated hypomethylated sites exist in the major breakpoint cluster region (M-bcr) where most Philadelphia chromosome (Ph) breakpoints are located. Twenty of 50 (40%) chronic myeloid leukemia (CML) patients were found to have aberrant hypermethylation of these sites on the rearranged M-bcr when compared with control marrows. The aberrancy correlated strongly with M-bcr breakpoint location; 19 of 20 cases had breakpoints located 5' of the M-bcr Sca I site, and 28 of 30 cases with normal M-bcr methylation had breakpoints located 3' of the M-bcr Sca I site. Sequence analysis of the Ph M-bcr breakpoints failed to find an M-bcr nucleotide position that delineated the transition between abnormally and normally methylated cases, indicating that the translocation of a critical M-bcr sequence was not responsible for the methylation abnormality. In 3 of 8 CML patients, cells without the t(9;22) were found to have abnormally methylated, unrearranged M-bcrs. The data indicate that abnormally methylated rearranged M-bcrs are present in CML cases with Ph breakpoints 5' of the M-bcr Sca I site and that the M-bcr in Ph- cells of patients with CML may also be abnormally methylated.