What Well-Dressed Beds Wear : Furnishings: European designers create sheet sets in silk and satin, a luxury that may cost $15,000 or more.

For $15,000, you can purchase a new car, fund a year at college, make a down payment on a condominium--or buy a set of sheets.

In the golden days of Hollywood, sheets made of black satin were considered the cat’s pajamas.

Today the ultimate in bed linens may be peach crepe de Chine with apricot embroidery (at a cost of $2,620 for the top sheet alone). Or the $15,000 beribboned and frilled set from the catalogue of D. Porthault & Co. For that price, it has been fashioned in France of the finest voile, decorated with fagoting, inset with satin ribbons and trimmed with handkerchief eyelet.

The wife of one Washington tycoon, who insisted on anonymity, ordered a similar style for their twin beds. With some custom changes to match her decor and a few throw pillows to boot, the set cost almost $20,000.

When money is no object, owning deluxe bed linens may be the ultimate self-indulgence. Unless the queen is coming to visit, they will be appreciated primarily by you and the laundry.

Call it the super sheet business, if you will. This is a world dominated by a handful of old-line European houses with names like Porthault and Pratesi, Leron and Descamps, Frette and Anichini. Even the priciest ready-to-sleep goods don’t come close: These are to Cannon and Fieldcrest what Maserati is to Volkswagen.

Athos Pratesi, owner of the prestigious Italian linens company of the same name, once boasted to a French magazine that most of Europe’s aristocracy was conceived on his sheets.

Jane Borthwick, the U.S. president of Porthault, went a step further in describing customer devotion: “We’ve been involved in divorce cases where there was a question of custody of the linens. Women may change their husbands, but they don’t change the sheets.”

In America, as in some 40 other countries, Porthault may be the best-known supplier of these luxury bed linens. The company applies the same finesse to its sumptuous table linens, towels, curtains, decorator fabrics, wallpaper and even covers for corporate jet bunks and pet baskets.

Porthault linens have been called “the most expensive in the world,” Borthwick notes. One consolation: The cost of the firm’s printed percales is only in the hundreds of dollars per sheet, not thousands. The company’s signature sheet--and its bestseller--is one with a floral design and a scalloped border in a contrasting color.

Founder Madeleine Porthault created the first flowered sheets in 1925. Since then her linens, according to Borthwick, have decorated many famous beds, among them those of Charlie Chaplin, the Duke and Duchess of Windsor, and Barbara Hutton. John F. Kennedy slept on Porthault sheets in the White House.

The company tradition, now in the hands of her son Marc Porthault, continues with such entertainers as Diana Ross, Lena Horne, Lauren Bacall and Jane Fonda. Barbara Walters is a customer. So is Walter Matthau, who played the untidy sportswriter Oscar Madison in “The Odd Couple.” He sleeps on linen and lace or flowery printed sheets in real life.

Even at $20,000 a set, the Washington tycoon’s custom sheets are a long way from being the world’s most expensive. That distinction probably belongs to a set of pure linen bed coverings made by Porthault and trimmed with three kinds of antique lace that sold for $45,000 last year.

Porthault’s record for the largest single sale to one individual on one day in New York was $145,000 cash for 20 sets of embroidered voile sheets and pillows--or $7,250 per set. Borthwick would reveal only that the buyer was female and an international celebrity in the entertainment business.

What makes these premium bed linens so expensive? After all, whether French or Italian, they are not handmade, but manufactured. Pratesi’s factory is in Pistoia, Tuscany; Porthault uses three factories in northern France. But the raw materials are of the highest quality: Frette, for example, uses linen from Belgium, Russia and Ireland, and cotton from Egypt.

Porthault’s cottons are woven at 260 to 320 threads per square inch. Then the entire process, from design to dyeing to hand-finishing, is controlled by the company. (Ordinary sheets are usually finished at a factory other than the one that looms the fabric.) Instead of selvage edge, sheets have scalloped borders; shams are embroidered in silk on four sides. In the end, it takes more time to turn out a Porthault sheet than a Detroit car.

Why do people buy super sheets? Listen to Betty Lou Ourisman, wife of Mid-Atlantic automobile magnate Mandell Ourisman: “The bed is so beautiful when it is made up. And these sheets last forever. A set improves with age like vintage wine. It gets softer and lovelier.”

Besides its repertory of 1,000 printed and 300 embroidered designs, Porthault specializes in fulfilling royal commands. Barbara Hutton, who would undoubtedly have been a duchess if America had a monarchy, commanded that tigers and elephants be embroidered on her sheets. Ex-King Edward VIII had his crown on his sheets.

All Porthaults are made of natural fibers to increase comfort. (It also increases ironing.) Like the princess in the fairy tale about the pea, everyone has a different comfort level.

Paloma Picasso, who once designed sheets, prefers percale over silk, which she feels creases badly and gets very warm. Linen is cool, but it should be ironed every day.

“When you don’t have good help, stick to cotton,” she advised.

Although Porthault lists among its customers designer Mario Buatta and actor Woody Allen, the average man may be more inclined to volunteer information to the Internal Revenue Service than to record his views on sheets.

When asked how her husband and partner, Rafael Lopez-Cambil, felt about her choices, Picasso commented, “It’s not fair to a husband to do (the bed) up in a very feminine way. Moreover, he can’t stand printed sheets. There is too much happening in a place when he is trying to get to sleep.”

“When most men get into bed,” said Ourisman, “I don’t think they give a damn what’s on the bed.”

On the other hand, Carol Matthau noted that her husband likes organdy and roses as well as classic prints.

Differences on fabric aside, everyone interviewed agreed on one aspect: These sheets improve with time. Twenty-year-old Porthaults are reputed to be at their peak in terms of sleep-ability. Longevity may also help lessen the impact of the investment. Over 20 years, the Washington tycoon and wife’s $20,000 purchase would come to $1,000 a year--or less than many car payments.

Some accessories get even longer wear. Matthau reports her husband treasures a Porthault bathrobe that is 30 years old. “Walter is meticulous, a very elegant man,” she said, adding, “Oscar Madison wouldn’t have been played that well by a slob.”

Borthwick says the sheets are rugged. She recommends laundering them in a machine with a mild detergent. Of course, some customers might have qualms about throwing into the washer sheets that cost five times as much as the machine itself.