The merger occurred in 1999, following the collapse of AHERF, and the health system struggled to maintain market share in Allegheny County and beyond. In the ensuing years, WPAHS lost market share to its rival UPMC, and WPAHS's relationship with Highmark Inc., the region's largest health insurer, also deteriorated: In April 2009, WPAHS filed an anti-trust suit against both Highmark and UPMC, “alleging that Highmark and UPMC illegally raised prices for local health consumers while trying to ‘destroy’ West Penn Allegheny. The suit detailed actions by the two going back to 2002 that it said were designed to assure Highmark's ongoing dominance in the local insurance market while establishing higher payment rates and eliminating West Penn Allegheny to benefit UPMC.”