This study aimed to describe adjuvant endocrine therapy adherence trajectories over 5 years and to identify factors associated with these trajectories, in a nationwide French cohort of breast cancer survivors. Among the 33,260 women included, 23,202 (68.8%) had a continuous optimal adherence trajectory. The main factors associated with nonadherence trajectories were extreme age (younger than 50 and older than 70 years) and switching adjuvant endocrine therapy. Background: Adjuvant endocrine therapy (AET) improves long-term survival of breast cancer patients, yet many women are nonadherent or discontinue this treatment. In this study we aimed to describe AET adherence trajectories over 5 years after treatment initiation and to identify factors associated with these trajectories, in a nationwide French cohort of breast cancer survivors. Patients and Methods: Every woman diagnosed with a first nonmetastatic breast cancer in 2011 in France who initiated AET in the 12 months after surgery was included from the French cancer cohort. We identified all reimbursements for AET from national health administrative data sets and modeled AET adherence trajectories over 5 years, using group-based trajectory modeling on the basis of the monthly proportion of days covered by AET. Associated factors were identified using multinomial logistic regressions. Results: We included 33,260 women. A 6-trajectory model was selected: 1, immediate discontinuation (6.6%); 2, continuous suboptimal adherence (4.3%); 3, progressive nonadherence then discontinuation (6.3%); 4, early nonadherence then discontinuation (5.7%); 5, continuous optimal adherence (68.8%); and 6, late nonadherence then discontinuation (8.3%). The main factors associated with nonadherence trajectories were extreme age (younger than 50 and older than 70 years) and switching AET. Conclusion: Approximately 70% of women had optimal adherence over all 5 years. The original nationwide approach enabled us to identify the "continuous suboptimal adherence trajectory" never previously described.