Changes in major angiogenic proteins have been seen following initiation of tamoxifen and aromatase inhibitor therapy in women with breast cancer. One source of these proteins is the circulating platelet pool. The investigators hypothesize that in addition to their anti-platelet properties, agents such as aspirin can be used as targeted anti-angiogenesis therapy. The investigators will determine the influence of aspirin on the release of angiogenic proteins from platelets in 35 patients with breast cancer.

Inclusion Criteria:

• Histologically or cytologically proven breast cancer
• Pre or post-menopausal
• Age >18
• Completed adjuvant non-hormonal therapy >30 days prior to initiation of study (surgery and/or chemotherapy and/or radiation therapy)
• Platelet count and hemoglobin within normal ranges for local lab within 30 days of initiation of study therapy
• Receiving tamoxifen therapy for at least 90 days prior to initiation of study therapy, and is expected to continue that therapy for the duration of the study (75 days)

Exclusion Criteria:

• Chemotherapy, radiation therapy or surgery within 30 days of study therapy
• Current use of aspirin, anti-platelet or anti-coagulation agents on a continuous basis
• Prior history of gastrointestinal or central nervous system bleeding, or documented or self-reported blood in stools or bright red blood per rectum