Background : Global DNA hypomethylation may result in chromosomal instability and oncogene activation, and as a surrogate of systemic methylation activity, may be associated with breast cancer risk. Methods : Samples and data were obtained from women with incident early-stage breast cancer (I–IIIa) and women who were cancer free, frequency matched on age and race. In preliminary analyses, genomic methylation of leukocyte DNA was determined by measuring 5-methyldeoxycytosine (5-mdC), as well as methylation analysis of the LINE-1-repetitive DNA element. Further analyses used only 5-mdC levels. Logistic regression models were used to estimate odds ratios (ORs) and 95% confidence intervals (CIs) for risk of breast cancer in relation to amounts of methylation. Results : In a subset of samples tested ( n  = 37), 5-mdC level was not correlated with LINE-1 methylation. 5-mdC level in leukocyte DNA was significantly lower in breast cancer cases than healthy controls ( P  = 0.001), but no significant case–control differences were observed with LINE-1 methylation ( P  = 0.176). In the entire data set, we noted significant differences in 5-mdC levels in leukocytes between cases ( n = 176) and controls ( n  = 173); P value < 0.001. Compared with women in the highest 5-mdC tertile (T3), women in the second (T2; OR = 1.49, 95% CI = 0.84–2.65) and lowest tertile (T1; OR = 2.86, 95% CI = 1.65–4.94) had higher risk of breast cancer ( P for trend ≤0.001). Among controls only and cases and controls combined, only alcohol intake was found to be inversely associated with methylation levels. Conclusion : These findings suggest that leukocyte DNA hypomethylation is independently associated with development of breast cancer.