AbstractHuman breast cancer can be divided into a group that contains specific receptor sites for estrogen and a group without such specific estrogen‐binding sites. The presence of specific estrogen receptors in some tumors indicating hormonal dependency has been shown to be of predictive value for endocrine treatment. This would greatly improve therapeutic planning for patients with breast cancer.Tumor tissue from 52 patients was investigated for content of both cytosol estrogen and estrogen receptor. In addition, the total tumor estrogen was also determined in 14 of these tumors. The results of this investigation show two distinct groups: one group containing both estrogen receptor and estrogen and a second group with no receptor but with measurable amount of estrogen. Tumors with estrogen receptors have higher tissue levels of estrogen than tumors without specific estrogen receptor. Even in the absence of estrogen receptor, however, most tumor tissue examined contained a measurable amount of estrogen.