Insurance companies regularly enter agreements with hospitals on how much they will pay for particular procedures or treatments. Without such an agreement, patients potentially face higher out-of-pocket expenses.

J. Larry Heinike, president and chief executive officer of UPMC Horizon, said, "We sent them (Highmark) a letter advising them of our interest in trying to find a resolution. We're following a structure and a process while we're trying to resolve the relationship we have.''

No changes occur immediately, Heinike said. The current agreement expires June 30 and negotiations could continue until then, he added.

With 19 hospital systems, UPMC is the largest health-care provider in western Pennsylvania, while Highmark is the largest health-insurance carrier. Both are nonprofit organizations based in Pittsburgh. UPMC Horizon, which has hospitals in Greenville and Farrell, is part of the UPMC system.

In recent years, UPMC and Highmark have sparred on issues such as advertising and hospital mergers. UPMC Health Plan, a for-profit harm of UPMC, offers health-insurance packages and competes head-on with Highmark.

Two months ago UPMC's board learned its executives had begun talks with United Healthcare, a subsidiary of Minnesota-based UnitedHealth Group, about selling the Pittsburgh insurance subsidiary or forming a joint venture.