For many years, hypersecretion of estrogens has been suspected of being one of the major risk factors of breast cancer for premenopausal women. Seventeen premenopausal women, who had undergone lumpectomy because of breast cancer (T1a No Mo) 3 yr before entering the study, were compared to 9 normal women of similar age, parity and body weight. A chemiluminescent method was used for the determination of estrone-3-glucuronide (E1-3G) and pregnanediol-3-glucuronide (Pd-3G) in early morning urine samples collected for an entire menstrual cycle of each of the 26 subjects. During the follicular phase, no significant differences in E1-3G and/or Pd-3G excretion were found between the two groups. During the luteal phase the E1-3G/Pd-3G ratio in the early, middle and late luteal phase had significantly increased in the women with breast cancer, in spite of normal Pd-3G excretion. Therefore, the measurement of glucuronoconjugate metabolites of ovarian hormones in overnight urine might be conveniently applied to the study of ovarian function in subjects with breast cancer. Furthermore, the results of this study may indicate that an estrogen/progesterone imbalance is an additional risk factor for the premenopausal breast cancer patient.