In addition to classic genetic mechanisms such as deletions and mutations, growth regulatory genes can be inactivated via methylation of cytosine-residues in their promoter regions. Hypermethylation of promoter CpG islands is now recognized as an important and early event in carcinogenesis. Detection of methylated DNA in serum or plasma has been suggested to be a marker for early cancer development. We examined methylation changes in RASSF1A, a growth regulatory gene in plasma DNA from blood collected before diagnosis from women with breast cancer and from controls. Samples were from two sets of subjects, 28 women with breast cancer and 10 of their unaffected siblings, and 33 women with breast cancer and 29 age- and ethnicity-matched population-based controls. Using methylation specific PCR, we found 11 of 61 (18%) cases were positive for methylation of RASSF1A in their plasma DNA collected before diagnosis. Two of 10 healthy high-risk sibling controls (20%) had plasma DNA positive for RASSF1A methylation in their plasma DNA compared with 0 of 29 (0%) population-based controls. Tumor tissue was available for 12 cases and all were positive for RASSF1A methylation. These results, if replicated, suggest that aberrant promoter hypermethylation in serum/plasma DNA may be common among high-risk women and may be present years before cancer diagnosis.