Background: The NCCN Clinical Practice Guidelines in Oncology (NCCN Guidelines) have directed the care of patients with cancer for >20 years. Payers are implementing guideline-based pathway programs that restrict reimbursement for non–guideline-based care to control costs, yet evidence regarding impact of guidelines on outcomes, including mortality, Medicare costs, and healthcare utilization, is limited. Patients and Methods: This analysis evaluated concordance of first treatment with NCCN Guidelines for women with de novo stage IV metastatic breast cancer (MBC) included within the SEER-Medicare linked database and diagnosed between 2007 and 2013. Cox proportional hazards models were used to evaluate the association between mortality and guideline concordance. Linear mixed-effects and generalized linear models were used to evaluate total cost to Medicare and rates of healthcare utilization by concordance status. Results: We found that 19% of patients (188/988) with de novo MBC received nonconcordant treatment. Patients receiving nonconcordant treatment were more likely to be younger and have hormone receptor–negative and HER2-positive MBC. The most common category of nonconcordant treatment was use of adjuvant regimens in the metastatic setting (40%). Adjusted mortality risk was similar for patients receiving concordant and nonconcordant treatments (hazard ratio [HR], 0.85; 95% confidence limit [CL], 0.69, 1.05). When considering category of nonconcordance, patients receiving adjuvant regimens in the metastatic setting had a decreased risk of mortality (HR, 0.60; 95% CL, 0.43, 0.84). Nonconcordant treatments were associated with $1,867 higher average Medicare costs per month compared with concordant treatments (95% CL, $918, $2,817). Single-agent HER2-targeted therapy was the highest costing category of nonconcordance at $3,008 (95% CL, $1,014, $5,001). Healthcare utilization rates were similar for patients receiving concordant and nonconcordant treatments. Conclusions: Despite a lack of survival benefit, concordant care was associated with lower costs, suggesting potential benefit to increasing standardization of care. These findings may influence policy decisions regarding implementation of pathway programs as health systems transition to value-based models.