Fall Fashion. Sportswear sets the tone in a lineup that includes short, fitted skirts and pattern mixing

SPORTSWEAR takes the lead again. It's been sportswear all the way -- or almost -- here at the American fashion presentations. The new offerings at the advance fall-winter showings, which extend over a two-week period, have that familiar relaxed yet luxurious-looking air. Not that the latest concepts are by any means all of a piece. They vary a lot, since not every designer takes the same approach, and in general the proportions, patterns, textures, and colors have undergone some marked changes from last year.

The most notable difference observed during the first week was the crop of short, fitted skirts that have sprouted all over Seventh Avenue. Short means well above the knee, sometimes mini, and fitted means straight and narrow, with perhaps a side or back slit for ease in walking -- or perhaps not, as with the more abbreviated skirts.

There are, to be sure, a few alternatives for the fainthearted: classic trousers or the newer, leaner stirrup pants, for instance. But the gathered skirt is now a sometime thing. So much was clear at Geoffrey Beene, where full skirts, which used to be a signature, have fallen out of favor. He includes only two of them, and showed no pants at all. His slim little skirts, in wool or leather, are worn with dark stockings and high heels and topped with carefully detailed jackets of various lengths, bolero to finger tip, often with dolman sleeves and moderate shoulders.

Styles are more evenly apportioned at Anne Klein & Co., and they are perhaps more representative of sportswear '85. Besides little skirts and pants, some flaring midcalf skirts appeared on the runway. The collection, the first solo for Louis dell'Olio (his former co-designer, Donna Karan, now has her own business), is a brilliant modernization of the man-tailored look for which this firm is famous.

Dell'Olio has touched all the bases. Suits that play off a glen plaid jacket against a complementary checked skirt make the point of pattern-mixing, a point well taken these days. Coats, which are getting a lot of fashion attention for next season, come in a variety of choices, from ankle-length reefers to button-out lining types, both long and three-quarter. Shoulders are strongly accentuated (as they are in Danny Noble's and the Blassport menswear looks), even on the blousy textured-knit cardigans. Colors are rich (a deep ruby red is memorable), and prints, besides giant paisleys, include a black and white pony-skin look-alike. The pony-skin print is part of the designer's Western-inspired styles, which drew rounds of applause. He accessorized the Urban Cowgirl Revisited group with Stetsons, stitched leather belts, and other engaging accents of Americana.

While the Anne Klein collection concentrates on black and white, olive, sapphire, and dark red, pastels are big elsewhere. As the men's fashions by Danny Noble made their debut with his women's clothes, longhaired male models appeared in powder pink tweeds and fleeces that were similar to what the short-haired girls wore (signaling that androgyny is still with us). Shirttails invariably hang loose in Noble's layerings of vest over shirt over tunic over tights, all of which are often topped by a jacket or coat of another length. The underlying principle, however, is practical, as one or another garment may be easily shed.

Pastels and pattern mixes are also in evidence at Blassport, where, besides banker's gray or camel, there are aqua, rose, and lemon brightening the fall palette. Bulky shearling coats, shown as evening coverage, come in these dulcet hues. The big blazers rather overwhelm the little skirts. What looks better are the windowpane-plaid intarsia sweaters with floral inlays at the yoke. These are matched with skirts or pants of the plaid.

The cabbage rose is a leitmotif in fall-winter prints, and, along with some beguiling red tartan silks, it appears at Albert Nipon in red-on-red crepe de Chine used for eminently wearable dresses with the characteristic pleats and tucks. The big roses also figured in patterned knits and printed satins at Adri, combined with plaid wool, mohair, and tweed. Adri's view of parts dressing is unconventional and utterly casual, a sense of ease being a primary concern. Antiqued panne velvets paired with paisleys and Lurex-rimmed mohair sweaters are among the breaks with tradition for late day. For evening comfort she showed pale jersey tank dresses over matching tights with, of all things, sneakers bought at Woolworth's and dyed to match.

No such shenanigans go on at Harriet Winter's, but the message there is also out of the ordinary. Her label (Mrs. H. Winter) is an assurance of distinction. The long, lean shape, extenuated by a hip yoke or a low-placed bias-cut flounce, is balanced by big sleeves, and the line is almost invariably graceful. How often do you find that these days?

For another kind of dramatic impact, there is Hollywood's Bob Mackie, whose clothes you may know from watching Diana Ross and Carol Burnett. Liberally be-furred coats and strapless matte jersey slinks vied with flashing Liberace glitter. But even show-biz folk like to relax, and Mackie is right in the sportswear mood with his series of dolman-sleeved sweater dresses knitted with intarsias of big Garbo-like faces on the backs.