The midcentury is in vogue these days, but suddenly, many influential designers' take is more 'Happy Days' than 'Mad Men.' For coming seasons, designers are channeling the sort of clothes that Peggy Sue got engaged in: roomy skirts with volume to flounce and flat-soled sneakers. It's a sock hop: circle skirts, hand-held purses, pearls and saddle shoes.

Singer Taylor Swift recently was photographed pairing a blue Amen circle skirt with a pair of white Keds, worn sockless. The skirt, cut well above the knee, with no sign of poofy petticoats, and worn with a snug sweater, was a modern take on a trend that predates her by four decades.

Fashion's latest take on '50s style focuses on an innocent look that originated with high school sophomores. Coming-of-age films such as 'American Graffiti,' 'Grease' and 'Peggy Sue Got Married' immortalized the style, along with the all-too-distant era of bobby-socks optimism. Taken too far, the style has potential to look like a costume.

But most designers, such as Peter Copping at Nina Ricci and Alber Elbaz at Lanvin, have updated the look to avoid a retro feel and make it fitting for a modern age. Mr. Copping, who says he has always been drawn to the midcentury, dropped the waists of his skirts down toward the hips. 'I didn't want the flare from the waist