Analysis of Medicare cost and utilization data has been extensively documented, most notably by the Dartmouth Atlas, and has revealed significant variation from one region to the next. Similar analysis using commercial insurance data, however, has been lacking. This study, the first to consider commercial populations, examines regional cost variation, providing cost relativities for claims paid by commercial payers for particular hospital referral regions. Among other findings, the study highlights the importance of negotiated provider reimbursement as a factor in the nation’s healthcare cost. While Medicare sets provider reimbursement rates based on formulas and rules, commercial provider reimbursement is set by negotiation between the insurer and the provider. This means, among other things, that regions with low Medicare costs could have high commercial costs. An examination of commercial data alongside Medicare data is crucial for understanding the true nature of healthcare cost variation across the country.