The affiliation was approved by the governing boards of both the health care systems.
"Together with Centegra, Northwestern Medicine will explore opportunities to expand and enhance clinical care offerings to patients in Northern Illinois," Dean M. Harrison, president and CEO of Northwestern Memorial HealthCare, said in a statement Tuesday. "Our shared goal is to provide patients with greater access to breakthrough treatments and specialized care, close to where they live and work."
Centegra, based in Crystal Lake, has hospitals in Huntley, McHenry and Woodstock.
Centegra joins Chicago's academic health system anchored by Northwestern Memorial Hospital and affiliated with Northwestern University's Feinberg School of Medicine.
"This partnership will combine Centegra's community-based care with Northwestern Medicine's academic health-care expertise," said Michael S. Eesley, CEO of Centegra Health System. "This will give our patients enhanced access to the highest level of care and will support our associates' continued commitment to service excellence."
Centegra Health System also has about 30 outpatient locations in McHenry and Kane counties. Centegra employs 3,700 associates and its medical staff includes 175 employed and more than 600 independent providers.
The health system has been facing financial struggles. About a year ago, Centegra Health System began moving all acute inpatient care from Woodstock to hospitals in McHenry and Huntley.
Eesley said at the time that the repurposing of hospital beds was prompted by "a flattening of growth in the Woodstock market over the last couple of years." He said the health-care industry is evolving more toward outpatient care rather than inpatient treatment.
The move came after the health system lost roughly $40 million last fiscal year since opening its $233 million, 128-bed Centegra Hospital-Huntley two years ago.
Eesley attributed $10 million to $12 million of those losses to the Huntley hospital. Variables such as population declines and higher numbers of Medicaid/Medicare patients -- up from roughly 5 percent more than 10 years ago to about 14 percent -- have contributed to the system's losses, he had said.
Meanwhile, Northwestern has added several suburban hospitals to its footprint, including Central DuPage Hospital in Winfield, Delnor Community Hospital in Geneva and Lake Forest Hospital.