The purpose of this study is to evaluate fulvestrant in the preliminary stage of breast cancer treatment and assess the relationship between dose, exposure, degree of reduction in tumor markers, and efficacy in postmenopausal women with estrogen receptor positive disease.

Inclusion Criteria:

• Postmenopausal women defined as women who have stopped having menstrual periods
• Written informed consent to participate in the trial
• Biopsy confirmation of invasive breast cancer
• Evidence of hormone sensitivity
• Willingness to undergo biopsies

Exclusion Criteria:

• Any previous treatment for breast cancer
• Unwillingness to stop taking any drug known to affect sex hormonal status or a patient in which it would be inappropriate to stop.
• Any severe concurrent condition that would preclude surgery or that would jeopardize compliance with the study, e.g., uncontrolled cardiac disease or uncontrolled diabetes mellitus
• The presence of more than one primary tumor
• History of hypersensitivity to castor oil
• History of known bleeding disorders