Eight-hundred and thirteen patients were prospectively studied to examine the influence of family history and the prior use of exogenous hormones as covariables in the subsequent expression of estrogen receptor protein (ERP) in the primary tumor of patients with breast cancer. Cases were divided by menstrual status; there were 385 pre- and perimenopausal and 428 postmenopausal patients. The influence of prior exposure to estrogen replacement therapy (ERT) in postmenopausal patients or oral contraceptives (OC) in pre- and perimenopausal patients on tumor ERP was analyzed controlling for family history: none, first degree (1 degree, mother or sister), second degree (2 degrees, grandmother or aunt), or both 1 degree and 2 degrees relatives. The results showed no influence of the prior use of ERT in postmenopausal women on subsequent tumor ERP. Among pre- and perimenopausal women, those with a family history of breast cancer in only a 1 degree relative, showed a borderline significant association between prior OC usage and subsequent tumor ERP. The use of OC was consistently associated with ERP negative tumors (9/9) whereas of 29 patients who had no prior OC exposure 17 had ERP negative tumors (P = 0.04, Fisher's Exact Test). Analysis of the prior exposure to OC, verified with the primary care physician or pharmacist, showed that these patients first used OC at the mean age of 32.2 years, had used OC for a mean duration of 41.9 months and stopped OC use a mean of 79.5 months before being diagnosed as having breast cancer. These results suggest that in a subset of patients with breast cancer, and a first degree relative only who had breast cancer, prior exposure to OC may influence the subsequent ERP status of the tumor. This is not due to exogenous estrogen saturation of receptors as there was a long latent period between exposure and diagnosis. Alternative hypotheses as to the mechanism of selection of subsequent tumor ERP may be either inhibition of ERP positive preneoplastic or tumor cell clones early in the evolution of the tumor or early selection of a tumor capable of endogenous estrogen synthesis with receptor saturation.