EVERY summer Lake Como welcomes throngs of Europeans on holiday, eager to go boating, fishing, or just to sun themselves along the mountainous shoreline.But as the tourist season winds down in October, Lake Como becomes a suitably grand backdrop for some high-powered deal making in the fashion business. Flying in by helicopter, or arriving by limousine, top designers and buyers from around the world gather at "Ideacomo," Italy's premier fabric show. Here on the lush grounds of Villa Erba, former home of Italy's Visconte family, designers and fabric companies engage in complex negotiations - proposals and counterproposals - that in the end greatly influence the direction of fashion. The colors, prints, and fabric types chosen here eventually dominate the market. Unlike other European fabric shows, Ideacomo is an intimate, classy affair, reserved for elite designers and just over 100 Italian companies who boast the finest, most expensive natural fabrics in the world - particularly silk. Most of the companies are based in Como, making it the epicenter of Italy's textile industry. "There's a heritage here that produces designs and quality that no one else in the world has been able to capture," says Fred Gates, who attended the show and is chairman and co-owner of Serica, a men's neckware company in New York that produces handmade silk ties in the $85 to $125 range. "[Italian silk companies] have a certain pride in what they do that's spectacular." Despite Ideacomo's gala atmosphere - the fine dining, the parties, the fashion shows - an undercurrent of secrecy and surveillance is apparent. Packs of security guards roam the grounds and hallways of the Ideacomo exhibition hall, a sprawling glass-roofed "greenhouse" across from Villa Erba. IDs are checked and rechecked - a precaution against uninvited visitors and foreign "copyists" hunting for the latest creations, officials say. Inside the sunlit hall each company has a private room where samples of fabrics for the fall/winter season of 1992 hang from racks. Within the rooms are private booths or isolated tables where deals can be made in privacy. In the trend-oriented business of fashion, buyers are sensitive to wandering eyes - people curious about what colors "so-and-so" may pick this year. Famous designers make an appearance only after a cadre of their assistants have "scoped out" the show ahead of time. Bright colors and geometric prints were prevalent at the show, along with classic Italian paisleys, tartans, solid neutrals, blacks, and whites. "People aren't just looking for a certain fabric, but for a special finishing that they can't find anywhere else," says Luca Valenzano, export zone manager for Olmetex, which sells to such names as Donna Karan, Ralph Lauren, and Christian Dior. Besides fine silk and other natural fabrics ranging $15 to $59 a meter, the company offers microfiber, a Japanese-invented polyester that Olmetex has refined to imitate fine Italian silk or luxurious corduroy. One sample called "Lavanda" feels as delicate as silk but has the surface design of old, cracked leather. "How is it made?" a visitor inquires. "This is top, top, top secret!" Mr. Valenzano replies. For many designers, samples on the rack are simply starting points from which they get ideas for a new fabric they want to develop through an exclusive arrangement with the company. "For our more important customers, we combine our proposal with their proposals ... and make something special," says Mercedes Cristina Curti, area manager for Segalini, which specializes in rayon prints for men's shirts and women's blouses. Top customers include Daniel Hechter, Edward Burton, Benetton, and Marks & Spencer. A designer may like the texture but not the color of a certain sample, or the yarn quality but not the finishing. What ensues can be a vigorous exchange of ideas, which often continues by phone, by fax, or in private consultations at the mill after Ideacomo is over. That can be exasperating, given the two-week to one-month time limits that are sometimes imposed, Valenzano says. "But it's worth it ... when you do something really new, something creative." Who exerts the most influence in determining the direction of high fashion - buyers or fabric companies? "I think it's both the designers and the companies in the same proportion," says Ms. Curti. "Maybe for one or two customers we know what they want because we have a close relationship with them.... But normally, we change our mind and our customers change their minds a lot." "We have influence," Valenzano says, "because designers start with our products and they feel how the market is moving." At the same time, talking with designers "let's us feel what the trend for the season might be." Carlalberto Ambrosini, president of the Ideacomo fabric company consortium, sums it up this way: "The supplier gives ideas but also takes ideas. The best results are always coming out from the cooperation."