Increasing evidence indicates the importance of ascertaining the quantitative estrogen receptor (ER) and progesterone receptor (PR) content (in femtomoles per milligram cytosol protein) of primary breast cancers. Those values obtained from the tumors of 1,887 patients participating in National Surgical Adjuvant Breast and Bowel Project protocol B-09 have been analyzed to define (1) the distribution of tumor ER or PR according to patient age, (2) the distribution of tumor PR within a specific ER interval, and (3) the concordance of tumor ER and PR levels. The present findings indicate how predictive the knowledge of the amount of one receptor (e.g., ER) may be for estimating the amount of the other (PR), when the latter is unknown.