Forty-one women with breast cancer and 119 controls participated in a case-control study of the relation of endogenous sex hormones to breast carcinoma in premenopausal women. During the follicular phase of the menstrual cycle, one overnight urine specimen was collected. During the luteal phase, urine and blood specimens were obtained. 17 beta-Estradiol, sex hormone-binding globulin, progesterone, and prolactin were measured in plasma, whereas estrogen metabolites (estrone, estradiol, and estriol) and pregnanediol were assessed in the urine. Breast cancer was associated with high-plasma estradiol and prolactin and with low progesterone. Similar but weaker associations were observed for urinary estrogens and pregnanediol in the luteal phase.