Women with breast cancer treated with aromatase inhibitor (AI) therapy experience multiple concurrent symptoms or symptom clusters. Understanding of the symptom experience and identifying symptom clusters before and during AI therapy are important for the development of interventions to improve clinical outcomes.The aim of this study was to identify symptom clusters experienced by women with breast cancer treated with AI therapy from pre-adjuvant therapy up to 18 months of adjuvant therapy using a broad scope of symptoms assessment.Forty-seven symptoms were evaluated in postmenopausal women with breast cancer (N = 354) who received AI therapy or chemotherapy followed by AI therapy. Symptoms were assessed at four semiannual time points with the Breast Cancer Prevention Trial Symptom Checklist, Patient's Assessment of Own Functioning Inventory, Beck Depression Inventory-II, and Profile of Mood States Tension/Anxiety and Fatigue/Inertia subscales. Exploratory factor analyses were conducted at each time point to identify symptom clusters.Four stable symptom clusters (i.e., musculoskeletal, vasomotor, urinary, sexual) and three relatively stable symptom clusters (i.e., psychological, neurocognitive, weight) were identified across the 18-month follow-up period. The gastrointestinal symptom cluster only appeared at after six months of adjuvant therapy (postchemotherapy).This study helps us to better understand the most common symptom clusters over the first 18 months of adjuvant therapy among postmenopausal women with early-stage breast cancer. It is critical for health care providers to know the symptom clusters commonly experienced by women receiving AI therapy with or without chemotherapy and manage them properly over time.