The silver lining, perhaps, is that mergers that took place across state lines were not shown to have a demonstrable effect on prices, although Dafny stopped short of suggesting that such acquisitions were good for the overall health care system.

Studies that show mergers lead to higher prices should get the attention of anti-trust authorities, as well as the hospitals themselves, she argued.

"Sometimes when you raise a community's healthcare costs, you jeopardize the community because it puts a strain on local businesses and their ability to stay open. If you really think the only way to stay open is to use bargaining power to raise prices, that might not serve the best interests of the community," she said.

The chorus of skepticism of hospital mergers represents a dramatic shift from the conventional wisdom touted by business leaders and policymakers until very recently.

President Obama, whose landmark health law actually encouraged mergers, pointed to Medicare data that showed that big hospital systems often spent less on Medicare treatments than smaller ones. But a study in December based on private insurance data showed that many of the hospitals that earned plaudits for delivering care to elderly patients more efficiently were in fact much more expensive for non-Medicare patients.