OBJECTIVES: Illustrate potential budgetary impact of a risk-share agreement in a US health plan using a case study of certolizumab pegol (CZP) as treatment for moderate to severe rheumatoid arthritis (RA). METHODS: An economic model was developed to evaluate the budget impact to a hypothetical 50 million member health plan of adding CZP to a formulary.  The target population was patients naïve to TNF inhibitors. Model comparators included adalimumab, etanercept, infliximab, and golimumab. Comparable efficacy is assumed across therapies; discontinuation rates were assumed to differ by therapy, as evidenced by claims data.  Model parameters were estimated using published data, market research, and an administrative claims database analysis. The base-case included a risk-share component for which the health plan received a rebate for clinically non-responsive CZP patients at week 12. Non-responsive CZP patients were assumed to receive an alternative TNF-alpha inhibitor for the remainder of the 3-year model horizon. Analyses captured differences in drug acquisition and intravenous administration costs for the base-case and an alternate scenario assuming no risk-share agreement. Deterministic sensitivity analyses were also conducted. RESULTS: Assuming a risk-share agreement, per-patient per-month (PPPM) savings in years 1, 2, and 3 were $4.22, $2.26, and $2.26, respectively. Applied to a hypothetical health-plan of 50 million members, annual cost savings are $3.6M, $2M, and $2.1M in years 1, 2, and 3, respectively.  Without a risk-share agreement, PPPM savings were $3.27 in year 1 and $2.21 in years 2 and 3.  Model results were most sensitive to average monthly pharmacy costs of CZP and infliximab.  CONCLUSIONS: The analysis suggests that under a risk-share agreement, the addition of CZP to a US health plan formulary may result in cost savings to the plan. Additionally, even without a risk-share agreement, the addition of CZP may provide cost savings.