BACKGROUND  Henriksson (Br J Surg, 2008) reported that surgery plus medical management can be considered cost effective as a treatment of asymptomatic carotid stenosis in 65 year-old men. He assumed a surgery relative risk reduction of 65.5%, or approximately an absolute risk reduction (ARR) of 8% in this population. However, data for the comparator arm (medical management alone) were dated and did not reflect efficacies of current medical therapies.  OBJECTIVES: To analyze using Bayesian methods, asymptomatic carotid stenosis clinical trial data, and more current medical therapy data the probability of achieving this 8% ARR and an incremental cost per QALY of approximately $50,000 (US, inflated to 2013 dollars). METHODS: The outcome of interest from the clinical trials was the mean difference in the probability of any stroke or perioperative death between surgery (carotid endarterectomy [CEA]) and aggressive medical management (MM). The CEA data came from the Asymptomatic Carotid Atherosclerosis Study (ACAS) and the Asymptomatic Carotid Surgery Trial (ACST). The updated medical management data came from a systematic review published in the journal Stroke (Abbott, 2009). The Bayesian analysis employed a Beta-Binomial Model. RESULTS: The posterior distribution of the Bayesian analysis representing the ARR of CEA versus MM had a mean of 0.008 with an essentially zero probability of achieving the Henriksson assumption of 8% ARR. Using the mean of this posterior distribution, the resulting incremental cost per QALY exceeded $500,000 in 65 year-old men – a value unlikely to be considered cost effective in any country. CONCLUSIONS: Bayesian analysis allows the prediction of the probability that a treatment alternative exceeds a predefined threshold. A powerful feature of Bayesian analysis is the ability to incorporate additional and/or newer data. This newer data can drastically alter assumptions about the cost effectiveness of treatment alternatives.