OBJECTIVES: As adjuvant endocrine therapy in women with early stage breast cancer prolongs disease free and overall survival, this study determined treatment continuation and first treatment switch in breast cancer patients starting on tamoxifen and the determinants of discontinuation. METHODS: Patients with early stage breast cancer were selected from the Eindhoven Cancer Registry from 1998 to 2006 and linked to the PHARMO Network to select drug use during follow-up. Patients starting on tamoxifen were included in the study cohort. Continuous use of endocrine treatment was determined as the time between start and stop of therapy, allowing a 60 days gap between refills. The switch from tamoxifen to an aromatase inhibitor (anastrozole, exemestane or letrozole) was determined. Cox regression was used to identify independent determinants of discontinuation of any endocrine treatment (tamoxifen and/or aromatase inhibitor use) during five years of follow-up. RESULTS: A total of 1291 new breast cancer patients started on tamoxifen. Of those, 29% had a treatment switch to: anastrozole (n=203), exemestane (n=113) or letrozole (N=64) with a mean duration until first switch of 2.0 (SD=1.3) years. Of the patients followed for five years, 49% discontinued any endocrine treatment before the completion of five years. Multivariate analyses showed that discontinuers were less likely to be aged 50-69 years (versus ≥70 years; HR=0.74; 95%CI: 0.60-0.92). CONCLUSIONS: Only half of the breast cancer patients starting tamoxifen continued five years of endocrine treatment. Identification of patients at risk of discontinuation will assist in the development of interventions to improve adherence.