Estrogen receptor determination in metastatic breast cancer has been shown to be important in determining potential for response to endocrine therapy. One limitation to this procedure can be the availability of neoplastic material for receptor assay. This report describes patients in whom estrogen receptor status was determined using malignant cells obtained through routine posterior iliac crest bone marrow biopsy. The ability to obtain receptor data from percutaneous bone marrow biopsy specimens has proved to be of important in patients in whom other sites involved with tumor were not accessible.