The Cuban government cooperates with backdoor travelers; customs officials generally do not stamp the passports of Americans when they enter.

"All travelers are legal as far as we're concerned," said Miguel Alejandro Figueras, a Cuban tourism official.

U.S. sanctions limiting travel to Cuba have waxed and waned in the last four decades. Travel loosened up during the Clinton administration; it has tightened during the Bush administration.

Backdoor travelers risk penalties ranging from a warning letter to $65,000 fines.

"Castro uses travel-related dollars to bankroll his regime on the backs of the Cuban people," said Molly Millerwise, a Treasury Department spokeswoman. "The Bush administration is steadfast in its commitment to hasten the day when the Cuban people can enjoy the same free lives we enjoy in America."

But many backdoor travelers say free travel is their constitutional right. Backdoor travelers usually play down the hazards, but the U.S. government managed to ferret out about 500 of them between January and October of last year.

"If you do get caught, it's probably going to cost you $1,000 to $2,000, if you're represented by an attorney," said lawyer Bill Miller of Oakland, Calif., who has done pro bono work for about 100 clients who faced fines for visiting Cuba. "If you ignore it (the warning letter), then it will be sent to collection and you're going to owe $7,500, plus interest."