Postmenopausal, ER and/or PgR positive breast cancer patients eligible for adjuvant hormonal treatment (HT), who are admitted for treatment and follow-up at the participating sites, will be enrolled in the trial. Study subjects will be treated with standard adjuvant hormonal therapy for a total of 5 years or until disease relapse or until HT is permanently discontinued for other reasons.

The choice of endocrine treatment strategy (drugs to be used or their sequence, if the sequential option has been decided) is left at the discretion of the participating site. Adjuvant HT will commence after the completion of adjuvant chemotherapy and/or radiotherapy.

Inclusion Criteria:

• Postmenopausal status. ER and/or PgR status positive. Adjuvant hormonal treatment for 5 years according to standard practice has been decided by the treating physician. Values within normal limits for the blood coagulation parameters at baseline visit. Accessible for follow-up for the duration of the trial. Written informed consent.

Exclusion Criteria:

• Both ER and PgR negative primary tumor. Evidence of distant metastases (M1) Neo-adjuvant hormonotherapy. Chronic use of oral anticoagulants such as warfarin or acenocoumarol. Psychiatric disorders preventing proper informed consent. Concomitant malignancies except for adequately treated carcinoma in situ of the uterine cervix or basal squamous cell carcinoma of the skin.