Three sources of information are available pertaining to the effect of oral contraceptive hormones on the breast: 1) toxicity experiments in animals; 2) histologic examinations of breast tissue from women taking hormones; and 3) epidemiologic studies of women using hormones. Animals treated with hormones at doses equivalent to the human contraceptive level have not developed cancer of the breast at a greater than expected frequency. Monkeys, the one experimental animal with a reproductive cycle similar to humans, have not developed cancer during long-term hormone administration except for a single animal which likely represents a chance occurrence. In humans, histologic studies of breast tissue have failed to identify abnormalities which could be attributed to hormones with rare exceptions of secretory change indistinguishable from normal lactating breast. Epidemiologic studies have usually shown a decreased frequency of benign breast disease and neither an increase nor a decrease in carcinoma of the breast. The one exception to the latter statement is a possible increased risk of cancer in long-term users who have had previous surgery for benign breast disease.