High levels of serum p,p′–DDT predicted a statistically significant 5-fold increased risk of breast cancer among women who were born after 1931. These women were under 14 years of age in 1945, when DDT came into widespread use, and mostly under 20 years as DDT use peaked. Women who were not exposed to p,p′–DDT before 14 years of age showed no association between p,p′–DDT and breast cancer (p = 0.02 for difference by age).