AbstractAnalysis of estrogen and progesterone receptor proteins was carried out in 75 premenopausal and 79 postmenopausal patients with primary operable breast carcinoma who were treated from January 1983 to December 1984. The frequency of estrogen receptor protein positive/progesterone receptor protein positive (+/+); estrogen receptor protein negative/progesterone receptor protein negative (−/−); estrogen receptor protein negative/progesterone receptor protein positive (−/+); and estrogen receptor protein positive/progesterone receptor protein negative (+/−) was 40.5%, 30.5%, 23%, and 6% in premenopausal patients, respectively, and 52%, 24%, 2.5%, and 21.5% in postmenopausal patients, respectively (p<0.001). The mean positive estrogen receptor protein concentration (expressed as femtomoles per milligram of protein $ SEM) was significantly higher in postmenopausal patients (54±6) than in premenopausal patients (19±2) (p<0.005). The progesterone receptor protein values did not differ significantly between these two groups. The phase of the menstrual cycle was recorded at the time of surgery in the 75 premenopausal women. Maximum receptor positivity occurred in the secretory phase, however, this difference is not statistically significant, and our data suggest that there are no distributional differences between the phase of menses and positivity of estrogen and progesterone receptor proteins. Future studies which included analyses of circulating sex steroid levels and receptor proteins will provide a better understanding of complex hormonal regulatory mechanisms which exist in patients with breast cancer.