James, 69, checked into Cedars-Sinai Medical Center for treatment, forcing her and her Roots band to drop out of the B.B. King Blues Festival -- which kicked off Tuesday in Hollywood, Fla. -- E! News reported Thursday. It was unclear when she was hospitalized.
Chaka Khan will take James' place for most of the concert dates, organizers said.
James rocketed to fame in the 1950s with a string of hits, including "Wallflower (Dance with Me Henry)," "Good Rockin' Daddy," "All I Could Do Was Cry," "Tell Mama," and her signature ballad, "At Last."
For a time, she was addicted to heroin, which landed her in jail for petty crime, as well as a psychiatric hospital.
James made a comeback with her album, "Seven Year Itch." Moving into jazz, she scored the first of three Grammy Awards for 1994's "Mystery Lady: The Songs of Billie Holiday."