There was a time not long ago when a trip across the border from the United States to Canada was accomplished with a wink and a wave of a driver's license. Those days are over.

Take the case of 55-year-old Lake Tahoe resident Greg Felsch. Stopped at the border in Vancouver this month at the start of a planned five-day ski trip, he was sent back to the United States because of a DUI conviction seven years ago. Not that he had any idea what was going on when he was told at customs: "Your next stop is immigration.''

Felsch was ushered into a room. "There must have been 75 people in line," he says. "We were there for three hours. One woman was in tears. A guy was sent back for having a medical marijuana card. I felt like a felon with an ankle bracelet.''

Or ask the well-to-do East Bay couple who flew to British Columbia this month for an eight-day ski vacation at the famed Whistler Chateau, where rooms run to $500 a night. They'd made the trip many times, but were surprised at the border to be told that the husband would have to report to "secondary'' immigration.

There, in a room he estimates was filled with 60 other concerned travelers, he was told he was "a person who was inadmissible to Canada.'' The problem? A conviction for marijuana possession.

In 1975.

Welcome to the new world of border security. Unsuspecting Americans are turning up at the Canadian border expecting clear sailing, only to find that their past -- sometimes their distant past -- is suddenly an issue.

While Canada officially has barred travelers convicted of criminal offenses for years, attorneys say post-9/11 information-gathering, combined with a sweeping agreement between Canada and the United States to share data, has resulted in a spike in phone calls from concerned travelers.

They are shocked to hear that the sins of their youth might keep them out of Canada. But what they don't know is that this is just the beginning. Soon other nations will be able to look into your past when you want to travel there.

"It's completely ridiculous,'' said Chris Cannon, an attorney representing the East Bay couple, who asked that their names not be used because they don't want their kids to know about the pot rap. "It's a disaster. I mean, who didn't smoke pot in the '70s?''

We're about to find out. And don't think you are in the clear if you never inhaled. Ever get nabbed for a DUI? How about shoplifting? Turn around. You aren't getting in.

"From the time that you turn 18, everything is in the system,'' says Lucy Perillo, whose Canada Border Crossing Service in Winnipeg, Manitoba, helps Americans get into the country.

Canadian attorney David Lesperance, an expert on customs and immigration, says he had a client who was involved in a fraternity prank 20 years ago. He was on a scavenger hunt, and the assignment was to steal something from a Piggly Wiggly supermarket. He got caught, paid a small fine and was ordered to sweep the police station parking lot.

He thought it was all forgotten. And it was, until he tried to cross the border.

The official word from the Canadian Border Services Agency is that this is nothing more than business as usual. Spokesman Derek Mellon gets a little huffy when asked why the border has become so strict.

"I think it is important to understand that you are entering another country,'' Mellon says. "You are not crossing the street.''

OK, but something changed here, didn't it?

"People say, 'I've been going to Canada for 20 years and never had a problem,' '' Lesperance says. "It's classic. I say, 'Well, you've been getting away with it for 20 years.' ''

A prior record has always made it difficult to cross the border. What you probably didn't know was that, as the Canadian Consulate's Web site says, "Driving while under the influence of alcohol is regarded as an extremely serious offense in Canada.''

So it isn't as if rules have stiffened. But what has changed is the way the information is gathered. In the wake of 9/11, Canada and the United States formed a partnership that has dramatically increased what Lesperance calls "the data mining'' system at the border.

The Smart Border Action Plan, as it is known, combines Canadian intelligence with extensive U.S. Homeland Security information. The partnership began in 2002, but it wasn't until recently that the system was refined.

"They can call up anything that your state trooper in Iowa can,'' Lesperance says. "As Canadians and Americans have begun cooperating, all those indiscretions from the '60s are going to come back and haunt us.''

Now, there's a scary thought. But the irony of the East Bay couple's situation is inescapable. Since their rowdy days in the '70s, they have created and sold a publishing company, purchased extensive real estate holdings and own a $3 million getaway home in Lake Tahoe.

"We've done pretty well since those days,'' she says. "But what I wonder is how many other people might be affected.''

The Canadian Border Services Agency says its statistics don't show an increase in the number of travelers turned back. But Cannon says that's because the "data mining'' has just begun to pick up momentum.

"It is too new to say,'' he says. "Put it this way. I am one lawyer in San Francisco, and I've had four of these cases in the last two years, two since January. And remember, a lot of people don't want to talk about it (because of embarrassment).''

Asked if there were more cases, attorney Lesperance was emphatic.

"Oh, yeah,'' he says. "Just the number of calls I get has gone up. If we factor in the greater ability to discover these cases, it is just mathematically logical that we are going to see more.''

The lesson, the attorneys say, is that if you must travel to Canada, you should apply for "a Minister's Approval of Rehabilitation" to wipe the record clear.

Oh, and by the way, if you don't need to travel to Canada, don't think you won't need to clear your record. Lesperance says it is just a matter of time before agreements are signed with governments in destinations like Japan, Indonesia and Europe.

"This,'' Lesperance says, "is just the edge of the wedge.''

Who would have thought a single, crazy night in college would follow you around the world?