Last I'd heard they were in second place, but as of a year ago frequent flyer miles surpassed the US Dollar as the world's most circulated currency.
The gold standard of sterling is long forgotten and now the supremacy of the greenback has been surpassed. The world has a new global currency - airline frequent flyer miles, which have a greater total value than dollars, euros, pounds or yen.
Doled out by airlines to seasoned travellers, they are intended to be redeemed for tickets as a reward for loyalty. But since their invention in 1981, their popularity has spawned its own economy of trading schemes, charitable donations, enthusiasts and scams.
By the end of 2004, almost 14 trillion frequent flyer miles had been accumulated worldwide, worth between 1p and 6p apiece.
According to a new analysis by The Economist magazine, the global stock is worth more than $700bn (£370bn), more than all the US dollar bills in circulation, and streets ahead of Britain's £42bn of notes and coins.
Although I'm sure there are more dollars in electronic circulation than as actual bills, so it may be that frequent flyer miles are still in second place in total quantity. Plus, it seems unfair to compare frequent flyer miles only with bills considering that no one I know of has any frequent flyer miles in bill form.