Two of Chicago's largest hospital systems on Thursday approved plans to combine, creating a 16-hospital, $6.8 billion system that promises to change the competitive landscape of health care in the Chicago region.

Advocate Health Care, the state's largest hospital system, and NorthShore University HealthSystem, the dominant hospital chain in the North Shore suburbs, said Thursday they will merge to create Advocate NorthShore Health Partners in a deal that's expected to close in early 2015.

The blockbuster deal is the largest hospital merger in Illinois in recent years, a trend set in motion by the federal Affordable Care Act and fueled in part by declining hospital patient volume and tighter finances.

It will create the 11th-largest nonprofit health system in the country and by the far the largest in Illinois, with more than 45,000 employees and 4,438 hospital beds.

"This is a huge win for Advocate. It's an incredible coup to lock up NorthShore. It's a great market and it's a great system," said Jordan Shields, a vice president at Juniper Advisory, a Chicago-based investment bank that provides merger and acquisition services to hospital systems.

The deal, Shields said, "is going to shake people. What this does is change the gravity in the metropolitan area."

Combined, the new health system will serve an estimated 3 million patients a year at more than 350 facilities including hospitals, clinics, doctors' offices and outpatient centers.

Both health systems' boards voted unanimously this week to approve the deal, which also will unite two of the region's largest medical groups, giving Advocate NorthShore more than 2,000 employed physicians.

The deal still requires approval from the United Church of Christ, which is affiliated with Advocate; the Federal Trade Commission; and state regulators.

Under terms of the merger, both health systems will contribute six members of a 12-member board of directors. Jim Skogsbergh, president and chief executive officer of Advocate, and NorthShore CEO Mark Neaman will be co-CEOs of the combined enterprise.

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Advocate Health Care and NorthShore to merge