We investigated the relationship between hypertension and breast cancer using data from a large case-control study of women younger than 55 years. Among nulliparous women, there was little evidence of an association between hypertension and breast cancer. Among parous women, hypertension reduced the risk of breast cancer if it had been diagnosed at any time in their lives before the end of the most recent pregnancy (odds ratio = 0.73; 95% confidence interval = 0.59-0.92). Several earlier studies indicate that there is an association between hypertension during pregnancy and elevated levels of maternal serum alpha-fetoprotein. Thus, our results are consistent with the hypothesis that maternal exposure to alpha-fetoprotein during pregnancy protects women against the subsequent occurrence of breast cancer.