Human breast carcinomas (n = 232) were evaluated for estrogen and progesterone receptors (ER, PR) by immunohistochemical study and by cytosol steroid-binding assay (n = 185). The staining was scored (histoscore) by estimates of relative nuclear staining intensity and the percentage of positively stained carcinoma cells. Of the invasive ductal carcinomas 72% were ER-positive and 55% were PR-positive. The invasive lobular, intraductal, tubular, and mucinous carcinomas were the most frequent ER-positive tumor types, whereas comedo and medullary carcinomas only rarely contained ER. Progesterone receptor was most frequently present in intraductal, tubular, and mucinous carcinomas. Better differentiated tumors with lower histologic grade were significantly associated with high prevalence of immunohistochemically determined ER and PR (P less than 0.0001). Proliferative cell fraction, determined by DNA flow cytometric study (n = 63), was inversely related to ER (P = 0.03) and PR (P = 0.05) status. Aneuploidy was independent of ER or PR content.