To date, no study has attempted to correlate hormone levels between successive pregnancies in the same woman. Serum levels of total estradiol and total free estradiol have been measured between the eighth and the 17th weeks in the first and second full-term pregnancies of 34 white women participating in the Collaborative Perinatal Study, 1959-1965. Intraindividual interpregnancy Pearson's product moment correlations were calculated. Partial correlation coefficients (adjusted for gestational age) for log pregnancy total estradiol and total free estradiol were 0.78 and 0.73, respectively (p < 0.001). These findings provide evidence that levels of pregnancy estradiol are significantly and strongly correlated in successive pregnancies of the same woman. This phenomenon can provide an explanation for the higher concordance of breast cancer incidence between two sisters than between a mother and daughter in the familial pattern of breast cancer.