Demographic risk factors in sisters and daughters of 150 patients with breast cancer were compared to those of controls. Plasma hormone levels in 36 teen-age daughters of patients and 31 controls were also studied to ascertain whether an "abnormal" hormone pattern underlies these risk factors. The patients' sisters had, on the average, menarche four months earlier and first full-term pregnancy 12 months later than the controls. The patients' daughters did not show these differences -- apparently owing to low fertility in the patients with early menarche. The patients' daughters had higher 22d-day estradiol-plus-estrone levels than controls (24.4 vs. 19.1 ng per 100 ml, P less than 0.05). Sixth-day prolactin was also elevated (19.0 vs. 14.2 ng per 100 ml, P less than 0.05). About half the patients' daughters could clearly be distinguished from the controls' daughters by means of the sixth-day information on both estrogens and prolactin. Hypersecretion of these hormones may be important factors in breast cancer.