The relation between estrogen receptors (ER) in breast cancer and risk factors was studied in 456 Japanese patients. ER was shown to be positive in 55.3% (252/456) of patients. There was no difference in ER positivity between pre- and postmenopausal patients. In premenopausal patients, only the age at menarche and obesity showed some influence on the ER positivity of breast cancer. Among postmenopausal patients, on the contrary, ER(+) cancers were predominant in the patient groups that have been known to be higher in the risk of incidence of breast cancer. The factors included were the age at marriage, number of pregnancies, number of live children, and body weight. Of these, the body weight of patients was the strongest influence on the ER positivity in the postmenopausal patients. After excluding the effects of body weight, some of the reproductive factors such as number of pregnancies and number of live children were shown to be related to the ER status. These results may suggest the combination of lower incidence of breast cancer and lower percentage of ER(+) cancers in Japanese postmenopausal women as compared with the Western countries.