This paper evaluates the effectiveness of Peak Health Alliance, a public–private initiative in Colorado aimed at lowering health care costs for employers and enrollees by increased bargaining power through the formation of a health care purchasing alliance. Using 2017–2021 plan data provided by the Colorado Department of Regulatory Affairs: Division of Insurance, we use difference-in-differences, event study, and synthetic control methods to compare changes in premiums in counties where Peak operated to other counties in Colorado before and after its implementation. The results suggest that Peak was associated with an increase in insurer market power and led to a 13%–17% decrease in average premiums, depending on the empirical specification. We further assess mechanisms underlying these effects and find evidence that lower prices were the most likely mechanism behind the estimated effect of Peak. Study results provide insights about the future of such public–private partnerships and their potential effectiveness.