The purpose of this study is to assess clinical outcomes after Letrozole treatment according to the estrogen receptor expression in postmenopausal women with hormone receptor positive breast cancer.

This study aimed at evaluating following:

1. Validity Assessment:

   1. Primary End Point: To evaluate 5-year diseae free survival rate after Letrozole treatment according to the estrogen receptor expression in postmenopausal women
   2. Secondary End Point: To evaluate overall survival rate(OS) and Time to distance recurrence(TTDR) after Hormonal therapy in postmenopausal women with hormone receptor positive breast cancer

2. Safety Assessment: To evaluate all adverse events including serious adverse events after Letrozole treatment.

Inclusion Criteria:

1. Patients with estrogen receptor(+) and/or progesterone receptor(+)

2. Postmenopausal state was defined the following conditions, at least one of a, b

   1. Serum FSH ≥ 30 mIU/mL and Amenorrhea ≥ 1 year for below 55 years, over than 55 years
   2. Bilateral oophorectomy

3. Patients have undergone surgery of the breast cancer within 12 weeks and terminated chemotherapy after surgery within 4 weeks.

4. WHO(ECOG) performance status 0-2

5. Adequate haematological function, renal function, hepatic function.

6. No evidence of metastasis.

Exclusion Criteria:

1. Metachronous bilateral breast cancer.
2. Metastatic breast cancer (stage IV)
3. Other hormone therapy and Hormonal replacement therapy given within the previous 4 weeks, except for Estring, Vagifem, Estrogen Cream.
4. Patients with Child-Pugh grade C, serum creatinine>2xUNL
5. Patients with gastrectomy, small bowel resection, malabsorption syndrome and dysphagia.

Patients recruited into the study are postmenopausal women with estrogen receptor positive breast cancer.