BACKGROUND: The global burden of breast cancer (BC) is high, especially in advanced stages. CDK 4/6 inhibitors represent a paradigm shift in the treatment of advanced BC HR+/HER2-, given the clinically and statistically significant gain in overall survival associated with this new class of medications. Nevertheless, as an innovation, the incorporation of these drugs impacts healthcare budgets, requiring cost-effectiveness analyses for decision-making. The aim of this study was to evaluate the cost-effectiveness of ribociclib plus letrozole compared with palbociclib plus letrozole or letrozole as monotherapy for first-line treatment of postmenopausal women with HR+/HER2- locally advanced or metastatic BC (aBC) from a Brazilian private healthcare system perspective. METHODS: A model including progression-free survival (PFS), progressed disease, and death health states was used to simulate lifetime costs and outcomes. PFS and overall survival were derived from the MONALEESA-2 trial (lifetime horizon). Healthcare costs included drug acquisition and monitoring, subsequent therapies, adverse events, and end-of-life costs. Effectiveness was measured in quality-adjusted life-years (QALYs). Deterministic and probabilistic sensitivity analyses were performed. RESULTS: The total cost of treatment with ribociclib plus letrozole was USD 72,091.82 versus USD 92,749.64 for palbociclib plus letrozole. Total QALYs were 3.30 and 3.16, respectively. Base-case analysis showed ribociclib as dominant over palbociclib in first-line treatment of women with HR+/HER2- aBC, associated with cost savings and QALY gains. The total cost of treatment with ribociclib plus letrozole was USD 83,058.73 versus USD 29,215.10 for letrozole. Total QALYs were 3.84 and 2.61, respectively. Compared with letrozole, ribociclib plus letrozole was associated with an incremental cost of USD 53,843.64 and an incremental QALY gain of 1.23, with incremental cost-effectiveness ratio of USD 43,826.91 per QALY gained. CONCLUSIONS: As demonstrated by the cost-effectiveness dominance over palbociclib, ribociclib results in savings when used as first-line treatment in postmenopausal women with HR+/HER2- aBC, warranting incorporation in the private healthcare system.