As the credit storm has raged in recent months, insurance companies like A.I.G. have been better positioned than the nation’s banks and brokerage firms to weather it because accounting rules do not require insurers to mark the investments held in their long-term portfolios to market. Insurance companies like A.I.G. can hold their investments until they mature, riding out the ups and downs in the market for those assets.

But the moment it began trying to raise capital, A.I.G. had to open its books to potential investors who were likely to take a sharp pencil to the company’s portfolio values, analysts said. And with Lehman Brothers last week providing investors with a valuation for the same types of assets held by A.I.G., subprime and Alt-A mortgage securities, the investment bank’s marks can now be applied to the big insurer’s books.

As of the most recent quarter, for example, A.I.G. had $20 billion of subprime mortgages marked at 69 cents on the dollar and $24 billion in Alt-A securities valued at 67 cents on the dollar.

But Lehman officials on a conference call with investors last week said it was valuing similar subprime mortgage securities to those held by A.I.G. at 34 cents on the dollar; its mark on the Alt-A holdings was 39 cents. Those valuations suggest almost a $14 billion decline in A.I.G.’s holdings, after taxes, an amount representing 18 percent of the company’s book value.

Additional write-downs may also be required in A.I.G.’s collateralized debt obligations, which the company does mark to market because they are held in a short-term account known as available for sale. The company valued $42 billion in high-grade holdings at 75 cents on the dollar, while it marked another $16 billion in lower-rated obligations at 70 cents.

A spokesman for A.I.G., Nicholas J. Ashooh, said it was inappropriate to compare the markdowns of Lehman Brothers’ securities with those at A.I.G.

“We don’t think that’s valid, to look at somebody else’s portfolio markdowns and then infer what A.I.G.’s might be, because there’s so many variables,” Mr. Ashooh said, “what kind of risk is in the portfolio, what kind of collateral there is, and how the marks were calculated. We think we use a very thorough and conservative approach that includes third-party input and input from the rating services.”