Epidemiologic data were studied and found incompatible with the horizontal transmission of a human breast cancer virus through milk. Regardless of the patient's age at diagnosis of breast cancer, no evidence of an increased risk associated with breast feeding was found. In addition, increased familial risk of breast cancer occurred in both paternal and maternal lines. The familial aggregation of the disease seemed related to an altered state of estrogen excretion or metabolism; the mothers of breast cancer patients had the delayed age at first delivery and at menopause characteristics of the patients. A decreased risk of breast cancer was associated with hormone administration at natural menopause.