TriWest Healthcare Alliance Corp., a closely held company providing military medical services, is weighing its next move after unsuccessfully challenging a $20.5 billion UnitedHealth Group Inc. contract.

The Government Accountability Office, which arbitrates contract disputes, denied a protest from Phoenix-based TriWest Monday over the Pentagon's March decision to give the contract to Minnetonka-based UnitedHealth.

TriWest, with about 1,700 employees, must now decide whether to file additional challenges to the contract awarded to the nation's largest health insurer, or sacrifice the source of almost all its revenue.

"There's an awful lot at stake for TriWest, so they may want to go down fighting and they may win," said Sheryl Skolnick, an analyst with CRT Capital Group in Stamford, Conn. "Until the last possible avenue of protest is exhausted, I have a feeling this contract won't be finally awarded."

The GAO found that the Defense Department's evaluation of the competing bids "was reasonable," Ralph O. White, the GAO's managing associate general counsel for procurement law, said in an e-mailed statement. It denied TriWest's protest "on all counts," he said.

BLOOMBERG NEWS