The head of the region's second-largest health system is moving aggressively to try to block the merger of University of Pittsburgh Medical Center and Mercy Hospital, raising concerns with the U.S. Justice Department, the state attorney general and others.

The moves by Jerry J. Fedele, chief executive officer of West Penn Allegheny Health System, come as questions swirl about what a merger, announced a week ago, would mean for health care costs in the region.

Mr. Fedele argues that a merger would further UPMC's goal of creating a health care monopoly that would drive up costs by using its market share to demand better reimbursements from health insurers like Highmark Inc.

UPMC officials would not directly respond to the charge, but said they pursued the merger as a way to save the county's last Catholic hospital. The merger, which Mercy officials have said was initiated by them, primarily would benefit UPMC by giving the health system room for a needed expansion of hospital services.

Still, concerns remain about the cost impact of the merger, which Mercy and UPMC officials hope to conclude by the end of the year if they successfully clear a barrage of regulatory and other hurdles, including approval from Rome.

A report being released today by the Pennsylvania Health Care Cost Containment Council notes that charges for patients treated at UPMC's Presbyterian and Shadyside hospitals are consistently and considerably higher than the charges at Mercy.

For example, the study finds that UPMC Presbyterian Shadyside charges anywhere from 37 percent to 92 percent more than Mercy for patients with any of 30 common diagnoses.

Insurance companies rarely pay the sticker prices, which is what the report measured, acknowledged Christine Whipple, executive director of the local employer coalition Pittsburgh Business Group on Health.

So it would be wrong, she said, to suggest the spread in charges between UPMC and other hospitals precisely mirrors the difference in payments received by the hospitals.

But the report does raise questions, she said, about why the gap in charges is so large; whether the addition of Mercy to the UPMC stable might widen the spread between UPMC and other hospitals; and whether having UPMC control that much more of the market might drive costs up further.

"We have a highly concentrated market as it is," Ms. Whipple said.

"To continue to highly concentrate that market -- does that have the possibility of making someone too strong, or an entity so big that it can start to set prices?"

For now, the lack of transparency in what hospitals are paid by health insurers makes it difficult to judge the competing claims that have been made by Mr. Fedele and Robert Cindrich, UPMC's chief legal counsel.

Both have suggested that their advanced or "tertiary" hospitals receive lower reimbursements from Highmark than their competitor gets for providing the same service.

Mr. Cindrich has said he believes Mercy Hospital's reimbursements are better than UPMC's, as well.

That claim is doubted by some members of the business community, who question why insurers other than Highmark that have Mercy in their networks have had such trouble negotiating contracts to add UPMC Presbyterian and Shadyside.

Highmark, for its part, does not release information about payment rates to the hospitals, which means the cost containment council had to base its findings on the reported sticker prices for various procedures, not on the reimbursements the hospitals actually got.

Still, Michael Weinstein, a spokesman for the insurer, said the proposed merger would have no impact on the reimbursements received by UPMC because the two parties are operating under a 10-year contract that won't expire until 2012.

Beyond that, it's not clear what impact the merger might have, he said. "There are too many variables to know what impact hospital and health system consolidations have on a market place."

The report on hospital charges, which is being released today, notes that patients with respiratory failure who needed mechanical ventilation at UPMC Presbyterian Shadyside between October 2004 and September 2005 faced average charges of $95,676. Patients with the same diagnosis at Mercy saw average charges of $54,494.

The council's report also notes that on average, hospitals in southwestern Pennsylvania received about 33 cents overall in net patient revenue for every dollar they actually charged.

While UPMC's charges might look high compared with Mercy, they're not as high as charges at academic medical centers in Philadelphia, said UPMC spokeswoman Wendy Zellner.

"The charge for an abnormal heartbeat case is $80,394 at Temple University Hospital vs. $27,505 at UPMC Presbyterian Shadyside," she said. "As a major academic medical center, we typically are treating the sickest patients and the most complex cases, which is reflected in those charges."

One things seems beyond dispute: A merger with Mercy would make UPMC bigger.

Pennsylvania Department of Health statistics indicate that as of June 2005, UPMC controlled 2,410 staffed hospital beds in Allegheny County -- 42.9 percent of the countywide total. Adding Mercy's beds would have given UPMC a majority -- 52.2 percent.

UPMC also controls the county's two premier specialty hospitals -- Children's Hospital of Pittsburgh, which had 260 staffed beds last year, and Magee-Womens Hospital, which had 223 staffed beds. Taking those beds into account, UPMC would have controlled about 56 percent of all staffed beds in the county.

Considering UPMC's size in the market already, Mr. Fedele, the West Penn Allegheny chief, said leaders of Mercy Hospital had an obligation to undergo a more thorough process in finding a merger partner.

"Mercy Hospital is a terribly important community asset," he said. "To say, 'We've decided we're going to cease doing business as Mercy Hospital' and default to the largest alternative that increases UPMC's quest for a monopoly, I think is inappropriate."