She died of empysema, according to CNN:
http://us.cnn.com/2004/SHOWBIZ/Music/02/19/obit.doris.troy.reut/index.html
Best known for the #1 1963 hit “Just One Look,” Troy was also a founding member of the Sweet Inspirations, a vocal group that provided backup on dozens of great soul, pop, and rock records that came out of the Brill Building and Atlantic Records during the early 1960s. (At various points members of the Sweet Inspirations also included Cissy Houston–Whitney’s mom–and Lee & Dee Dee Warwick–Dionne’s sisters.)
During the early to mid 60s Troy also released a number of singles under her own name. “Just One Look” is the most famous, but “I’ll Do Anything” is also excellent.
Troy is also known to rock fans for her backup singing on the Rolling Stones’ Let It Bleed and Pink Floyd’s Dark Side of the Moon. During the 80s she starred in Mama I Want to Sing, a musical based on her life.