OBJECTIVES: Pharmacoeconomic studies often require estimating the cost of drugs using manufacturer listed prices. There is a lack of studies assessing the relationship between actual prices and manufacturer listed prices. This study compared the average sales price (ASP) and the average wholesale price (AWP) of single source brand drugs reimbursed by Medicare Part B for drugs used in physician offices in 2012. METHODS: ASP data for the four quarters of 2012 were extracted from the Centers of Medicare and Medicaid Services (CMS) webpage, AWP data from the RedBook (Truven Health Analytics, Inc.) and drug information from the FDA website. Descriptive statistics and 95% confidence intervals were used in the analysis. RESULTS: CMS listed 571 products reimbursed by Medicare Part B in 2012. There were 243 (42.6% ) products with complete ASP and AWP information and without generic competition, of those  60.5% were new drugs applications (NDA), 22.2% therapeutic biologics (BLA), 12.8% other biologics, and 4.5% vaccines. The average ASP was 73.9%±34.5% the AWP in the first quarter of 2012, 72.4%±22.9% in the second quarter, 72.0%±24.5% in the third quarter, and 72.3%±26.5% in the fourth quarter. The relationship ASP/AWP in the fourth quarter was: 76.3%±89.0% (95%CI: 71.3%-81.3%, n= 147) for NDAs, 82.5%±9.6%  (95%CI: 80.0%-85.1%, n= 54) for BLAs, 75.4%±38.9% (95%CI: 68.2%-82.7%, n= 31) for other biologics, 64.6%±18.6% (95%CI: 53.6%-75.6%, n=11 )for vaccines, and 77.0%±70.7%  (95%CI: 73.7%-80.4%, n= 243) for all products combined. CONCLUSIONS: The use of AWP overestimates the actual manufacturer average sales prices. ASP represented approximately 72% of the AWP across all products. This relationship was larger could explain the differences between actual and manufacturer listed prices.