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One plausible mechanism for the environment to alter cancer susceptibility is through DNA methylation. Alterations in DNA methylation can lead to genomic instability and altered gene transcription. Genomic DNA methylation levels have been inversely associated with age, suggesting that factors throughout life may be associated with declines in DNA methylation. Using information from a multiethnic New York City birth cohort (born between 1959 and 1963), we examined whether genomic DNA methylation, measured in peripheral blood mononuclear cells, was associated with smoking exposure and other epidemiologic risk factors across the life course. Information on prenatal and childhood exposures was collected prospectively through 1971, and information on adult exposures and blood specimens were collected in adulthood from 2001 to 2007. Methylation levels of leukocyte DNA were determined using a [(3)H]-methyl acceptance assay where higher values of disintegrations per minute per microgram DNA indicate less DNA methylation. Genomic methylation of leukocyte DNA differed by ethnicity (66% of Blacks, 48% of Whites, and 29% of Hispanics were above the median level of disintegrations per minute per microgram DNA; P = 0.03). In multivariable modeling, DNA methylation was statistically significantly associated with maternal smoking during pregnancy, longer birth length, later age at menarche, nulliparity, and later age at first birth. These data, if replicated in larger samples, suggest that risk factors across the life course may be associated with DNA methylation in adulthood. Larger studies and studies that measure within-individual changes in DNA methylation over time are a necessary next step.