George, who had a private mass with bishops at his residence Wednesday, was elevated to the Sacred College of Cardinals in 1998 by Pope John Paul II, after becoming the first native Chicagoan to serve as archbishop of the city in 1997.
He reportedly told bishops he had cancer a few days ago and will have surgery at Loyola Medical Center Thursday, Auxiliary Bishop John Manz told WMAQ-TV, Chicago.
Doctors were expected to remove his bladder, WLS-TV, Chicago, reported.
George, 69, the eighth archbishop of Chicago, has had to deal with several stressful, headline-grabbing clergy sex abuse scandals in the archdiocese in recent years.
Ordained as a priest in 1963, George served in Rome from 1974 to 1986 when he was named archbishop of Yakima, Wash. He became archbishop of Portland, Ore., before returning to Chicago and being elevated to Cardinal.