Google's 5 percent stake in Time Warner's AOL unit may be worth less than the $1 billion the Web company paid for it in 2006, Google warned in a regulatory filing on Thursday.
"We believe our investment in AOL may be impaired," Google said in its latest quarterly financial filing with the U.S. Securities and Exchange Commission.
Google said it would continue to review its investment for impairment, and financial write-downs could be required in the future.
In a deal announced in December 2005 and which closed the following year, Google paid $1 billion in cash for a 5 percent indirect equity stake in AOL.
The deal by the Mountain View, California-based company gave AOL a theoretical valuation of $20 billion at the time.
In return, Google secured renewal of its search advertising deal with AOL, its largest ad partner, at least until Google's recent partnership with Yahoo takes effect in coming months, analysts say. Google's original pact with AOL in 2002 was the landmark deal that legitimized Google's advertising services.