CHICAGO — It has six full baths, 259 feet of Lake Geneva shoreline and a price tag that thumbs its nose at the recent slowdown in the residential real-estate boom.

For sale at $10.4 million, this gated mansion complete with boat dock and billiard room is testing the underlying strength of a market that some expect to be the first to cool: second homes.

"It's a fabulous, fabulous house," listing agent Nancy Lehman attests. But it may be too fabulous for the times, some experts say.

As investors cast a wary eye on the housing market, conventional wisdom suggests that such desirable retreats will be early casualties. Wildly popular in recent years, these mansions, cottages and condos with their scenic views have come under pressure from simple economics, as the run-up in selling prices and interest rates suppress demand for a sought-after luxury.

"The issue is whether we can afford it," said Northern Trust chief economist Paul Kasriel. "It's getting more expensive."

And it's far from a necessity, noted Richard Peiser, a Harvard University professor specializing in real estate who considers second homes more vulnerable in a downturn. "It's still a discretionary part of a homeowner's budget," he said.

Yet despite widespread reports of slowing sales and moderating prices, certain second-home markets could display remarkable staying power. The growing economy is providing what Fannie Mae chief economist David Berson describes as "underlying strength."

And significant additional support, he said, will stem from an irresistible demographic phenomenon that already has helped make vacation homes and investment property more than one-third of all residential real-estate sales. Demand from baby boomers as they enter the prime years for second-home purchases is likely to help buoy prices for the next decade and beyond.

It's a theory fleshed out in the research of Kenneth Johnson, a Loyola University demographer who foresees a "tidal wave" of boomers migrating to scenic, rural byways within striking distance of the metropolitan areas where they currently live and work.

The sheer scale will buttress the market, Johnson said. "This is only the beginning. We're talking 18 years, 4 million people a year," he said. "There's only so much pretty land. This is a force to be reckoned with."

Second homes have been soaring for the past five years, as the stock-market bust gave way to a real-estate boom fueled by low interest rates.

Those buying properties for personal use make up a big part of the market. But across the country, an even bigger part has consisted of those seeking an investment return.

Some highly leveraged speculators have gotten squeezed when they were unable to realize the rents needed to offset their carrying costs. Others have been luckier, buying and selling in short order to exploit rising values.

In spots like Oceanside, Calif.; Pompano Beach, Fla.; and Beach Haven, N.J., seasonal homes have become some of the hottest commodities going. Prices have soared along both coasts, as well as in well-known ski areas and resort communities. "You see appreciation in unique areas that are simply irreplaceable," said Peiser.

In 2004, 1.8 million second homes were bought primarily for investment, according to the National Association of Realtors. That was 23 percent of all homes purchased in 2004.

Another 1 million second homes changed hands for personal use, or 13 percent of the total. Taken together, all those Donald Trump wannabes and cocooning baby boomers accounted for 36 percent of home sales overall.