The extent to which concentration in the health insurance market affects negotiated prices paid to hospitals is of high interest to policy makers. We examined the association between insurer market share and hospital prices, using a new source of data obtained through the federal Hospital Price Transparency initiative. We found that the market-leading insurer in the least competitive (most concentrated) insurance markets pays 15 percent less to hospitals than the market-leading insurer in the most competitive (least concentrated) markets. We also found the price relationship to be more pronounced for for-profit hospitals than for not-for-profit hospitals. Our results invite the question of whether dominant insurers are passing savings on to employers in the form of lower premiums.