WHAT may be the most important number in the American panoply of economic statistics appears every Thursday night as an appendix to the weekly statement of the condition of the Federal Reserve System. This generally ignored number -- few, if any, newspapers cover its release -- has the unusual virtue of accuracy, for it is a simple financial statement derived from an adding machine, not from a computer or a formula.
What the number announces is the quantity of government and agency securities held ''for foreign official and international accounts'' -- that is, for foreign central banks and finance ministries -- by the federal reserve banks. It is important because over time it measures the demand for American assets by private enterprise in the world's creditor nations. It is important also because it is very large -- last week, about $1.63 trillion. Three years ago, just before the invasion of Iraq, it was about $900 billion. The week George W. Bush took office, it was $693 billion.
Our appetite for imported goods throws some $600 billion to $700 billion a year into the hands of foreign suppliers. The businesses that receive these dollars have two fundamental choices about what to do with them: spend or invest them in the United States, or convert them into their own local currency.
Exporters to America who keep the dollars and use them for American purchases and investments create what economists call an autonomous flow of funds back to the United States, financing the American trade deficit with an American investment surplus.
This produces the argument most closely associated with the new Federal Reserve chairman, Ben Bernanke (though Alan Greenspan believed it, too), that our trade deficit is caused by a surplus of savings that can't be profitably invested in the home countries of our trading partners. Financing for our trade deficit comes before -- and actually causes -- the deficit itself.
If instead of investing their dollars in the United States, foreign exporters want to take the proceeds of their sales in their own currency, their central banks will in effect sell them that currency for their dollars. Back in the late 1960's, when Great Society deficits and the Vietnam War prompted the first serious sell-off of dollars (and forced the United States to abandon the gold standard because too many holders of dollars, led by President Charles de Gaulle of France, wanted gold), those central banks lent those dollars into the new Eurodollar market, where they traded somewhat separately from domestic dollars.
This created a nightmarish prospect of the United States losing control of its own currency, and in 1971 the Fed chairman, Arthur Burns, negotiated a deal with the European and Japanese central banks. The deal was that they would return to America the dollars they acquired in their own economies, and the Fed would invest the money on their behalf, in absolutely safe government securities, without charge and at the best rates.
Today, the Fed continues as custodian of the ''foreign official holdings'' of such government obligations. During the Clinton administration, the Fed agreed to invest in federally guaranteed housing securities for those foreign central banks that wanted a better yield on their dollar reserves than they would get from government bonds, and now more than half a trillion dollars of the total official holdings are invested in agency paper. Foreign official holdings of government paper is a miner's canary number. It tells you if there is big trouble ahead. The most common worry is that the number will shrink suddenly, with foreign governments dumping their dollar holdings, driving down the dollar's value and driving up American interest rates, but that's not a real danger. If the price of our government securities dived, the foreign central banks would have to bear the loss. This would be a budget item for their governments, whose leaders would not like it at all.
What we have to watch out for is a sudden and drastic increase in foreign official holdings. Rapid growth in this number in the late 1960's and 1970's forecast the recessions of the early 1970's and 1980's, and it could happen again.
Recent large increases in foreign official holdings indicate that foreign private investors see fewer attractive places to put their money in the American economy. They could presage a significant fall in the price of American assets, stocks (witness the recent drops in American stock markets) and bonds and real estate and all, and a hard landing for a world economy still floating on the crest of cheap credit.
Martin Mayer, a guest scholar at the Brookings Institution, is the author of ''The Fate of the Dollar.''