BACKGROUND: The 2015 American Society of Clinical Oncology guidelines recommend first-line treatment of hormone receptor (HR)-positive breast cancer with endocrine therapy plus or minus palbociclib, a selective cyclin-dependent kinase (CDK)4/6 inhibitor. In 2018, the U.S. Food and Drug Administration approved ribociclib, a new orally available selective CDK4/6 inhibitor. While gains in progression-free survival (PFS) and overall survival (OS) from ribociclib are important for clinical and treatment outcomes, trade-offs in adverse events (AEs) and additional costs necessitate cost-effectiveness analysis (CEA) to assist consideration by third-party payer systems, physicians, and patients. OBJECTIVES: To (a) develop a Markov model and (b) determine the cost-effectiveness of ribociclib plus endocrine therapy versus endocrine therapy alone as treatment for premenopausal and perimenopausal patients with HR-positive, human epidermal growth factor receptor 2 (HER2)-negative breast cancer. METHODS: A lifetime 3-state Markov model (“stable,” “progressed,” and “dead” health states) was developed using a U.S. payer perspective. Transition probabilities were calculated based on OS and PFS outcomes from the randomized controlled phase 3 trial MONALEESA-7. These Kaplan-Meier curves were extended to lifetime by estimating best-fit distributions using loglogistic distribution for ribociclib curves and Weibull distribution for placebo curves. Costs were obtained from national data sources using 2019 U.S. dollars (USD) and discounted by 3%. Utilities were obtained via published breast cancer literature and were included for each health state and for time spent with each AE. Results were expressed as an incremental cost-effectiveness ratio (ICER) expressed as USD per quality-adjusted life-year (QALY) saved. Treatments were assumed to be cost-effective based on a willingness-to-pay (WTP) threshold of $150,000 per QALY gained. Base-case, 1-way sensitivity tornado diagrams and probabilistic sensitivity analyses demonstrated changes in the ICER and were driven by the cost of ribociclib and the utility of remaining in the stable health state. RESULTS: Ribociclib plus endocrine therapy was cost-effective at an ICER of $124,513 per QALY when compared with endocrine therapy alone at a WTP threshold of $150,000. The ribociclib plus endocrine therapy arm had an effectiveness of 5.28 QALYs and a total cost of $385,112, while placebo plus endocrine therapy provided only 2.46 QALYs at a lower total cost of $67.246. The model was sensitive to the cost of ribociclib and the utility of time spent in the stable health state. Probabilistic sensitivity analysis demonstrated that endocrine therapy alone was cost-effective until a WTP of $125,000 and was cost-effective 72% of the time at the WTP threshold. CONCLUSIONS: Ribociclib plus endocrine therapy is more cost-effective than endocrine therapy alone. Professionals in managed care settings should consider the pharmacoeconomic benefits of ribociclib for the treatment of HR-positive, HER2-negative breast cancer as they make value-based formulary decisions. Further CEAs should be considered as direct treatment comparison trials between CDK4/6 inhibitors are completed in the future. DISCLOSURES: No outside funding supported this study. The authors have nothing to disclose.