Zawinul, 75, was admitted to Vienna's Wilhelmina Clinic on Sunday, the Viennese Hospital Association said.
The group declined to confirm reports by Austrian public broadcaster ORF that Zawinul was seriously ill, saying the musician had requested that his rights as a patient and a private person be respected.
In 1961 he joined saxophonist Cannonball Adderly's group, for whom he wrote the hit track "Mercy, Mercy, Mercy."
Zawinul won widespread acclaim for his keyboard work on chart-topping Davis albums such as "In a Silent Way" and "Bitches Brew." He was a leading force behind the so-called jazz fusion movement, which focused on the use of electric guitar, bass and keyboards and "fused" elements of rock, pop and funk with jazz.
After that band's breakup, Zawinul founded the Zawinul Syndicate in 1987.
Zawinul is credited with bringing the electric piano and synthesizer into the jazz mainstream.
This past spring, he toured Europe to mark the 20th anniversary of the Zawinul Syndicate. He sought medical attention when the tour ended, the Hospital Association said.
His record label, BirdJAM, said there was no word on whether he planned to go ahead with concerts scheduled for next month.