Eletra Casadei, the California fashion designer whose prom dresses put away the Sweet Sixteen look and moved into strapless, backless, slit-to-there styles, has died. She was 55.

She died Sept. 27 at her home in the Los Angeles neighborhood of Pacific Palisades. The cause was brain cancer, said her sister, Andrea Casadei Best.

One of the first Los Angeles designers of her generation to gain a national reputation for something other than swimwear, Ms. Casadei claimed Old Hollywood glamour as her inspiration and fantasy dresses at affordable prices as her niche.

She introduced her TD4 (To Die For) line in the late 1970s for customers ages 14 and up. Before long, their mothers were wearing the clothes, and she launched a second collection, Casadei, for them. By the early 1980s, a boom time for flaunt-it fashion, Ms. Casadei was in her stride.

Throughout the 1980s, Ms. Casadei's collections were carried in some 7,000 boutiques and department stores, most of them in the U.S. Prices ranged from about $100 to about $400.

A former fashion model, she wore her own designs.

Dresses from her collections turned up on television sitcoms and soap operas as diverse as "Golden Girls" and "Dynasty."

Ms. Casadei's best advertisements were the two fashion-music videos she created in the mid-1980s, modeled after the music videos that aired on MTV. Instead of costumes, she used dresses from her latest collection.