ARTEMIS is a study to assess how educational material affects overall adherence to anastrozole by evaluation of patients' electronically compiled dosing histories. Patients' dosing times are compiled electronically using Medication Event Monitoring System" (MEMS®) in order to be able to gather accurate, objective and up-to-date patients' dosing histories. For subjects in Group A, the control group, the adherence to anastrozole in the standard clinical practice will be evaluated. For subjects in Group B, on top of the standard clinical practice, they will also receive educational material by mail at week 0 (study registration), week 2, week 4, week 6, week 8, week 12, month 5, month 7, and at month 10. This procedure is the same as in the ongoing CARIATIDE study. One of the reasons of poor adherence (either early discontinuation of the treatment or missing doses while still engaged to the therapy) can be some side effects experienced by the patients during the anastrozole treatment. Recording the reasons at discontinuation allows us to identify reasons for treatment discontinuation. Recording reasons of anastrozole interruptions during the study period allows us to relate these events to patients' adherence to anastrozole. During the study, no pro-active safety data collection will take place. Spontaneous reported adverse drug reactions (ADRs) will be reported according to local post-marketing pharmacovigilance regulations.

Inclusion Criteria:

• Postmenopausal women with hormone sensitive early breast cancer.
• Documented decision of treatment with anastrozole which will start in 13 weeks according to current SmPC OR current treatment with anastrozole according to current SmPC, that has not exceeded thirteen weeks before enrollment.
• Subjects who accept to use MEMS® monitors to automatically compile their drug dosing histories.

Exclusion Criteria:

• Concomitant or previous use of adjuvant tamoxifen, letrozole or exemestane for the current tumour.
• Involvement in the planning and/or conduct of the study (applies to both AstraZeneca staff or staff at the study site).
• Participation in another clinical study with an investigational product during the last 3 months.

Postmenopausal women with hormone sensitive early breast cancer