WASHINGTON (AP) -- Globe-trotting members of Congress reap a valuable fringe benefit they do not disclose: frequent-flier miles from trips they take at the expense of special interests or taxpayers.

It does not take long for the miles to add up for free personal travel or upgrades to first class.

''There's no question it's a definite benefit. I would call it a nice perk,'' said Rep. Ray LaHood, R-Ill. He uses the frequent flier miles for upgrades and personal free trips, such as travel to charity golf tournaments in Sun Valley, Idaho.

LaHood and his wife each accumulated about 13,500 miles this year from a round trip between Chicago and China financed by the Aspen Institute. LaHood was among a dozen lawmakers attending a conference on U.S.-China relations sponsored by the Washington-based think tank.

Lawmakers routinely travel at the invitation of private groups or on official trips for their congressional committees. Frequent flier credits are not part of the information they must report about the trips or disclose on their annual ethics statement.

That lack of disclosure baffles even some groups that pay for the trips.

''Eventually a couple of trips is a free trip, right? So I suppose taking into account how rigid the normal rules are, this should technically be accounted for,'' said Kiran Pasricha, who heads the Washington office of the Confederation of Indian Industry. The pro-trade group, financed by Indian companies, flies several members of Congress to India each year.

Consumer Electronics Association spokesman Jeff Joseph, whose group pays for congressional visits to Las Vegas for its convention, said: ''It does seem a bit strange that members have to report what really are nominal gifts ... but not report frequent flier miles.''