The epidemiological and clinical evidence for various forms of exogenous estrogens altering the risk of neoplasms of the female genital system, breast, and liver are reviewed and evaluated. It is virtually certain that in utero exposure to diethylstilbestrol can cause clear cell adenocarcinomas of the vagina and cervix. There is strong evidence that various estrogens given for treatment of menopausal symptoms can cause endometrial carcinoma and that sequential oral contraceptives probably also do so. Oral contraceptives very probably reduce the risk of both cystic disease and fibroadenoma of the breast and increase the risk of liver cell adenomas. Studies to date do not provide consistent and convincing evidence that any form of exogenous estrogen alters the risk of cancers of the breast or ovary or that oral contraceptives alter the risk of cervical neoplasia or focal nodular hyperplasia of the liver, although recent reports suggest that continued vigilance is warranted. Specific topics requiring further epidemiological investigation are suggested.