They prayed for the cardinal during mass at Holy Name Cathedral Saturday.
Doctors found cancer cells in George's kidney and liver according to a statement from the Chicago Archdiocese.
George underwent surgery six years ago for bladder cancer.
He has served as Chicago's archbishop for 15 years.
In January, he submitted his request to retire to the pope after he reached the mandatory retirement age.
The Vatican has not yet made a decision on his resignation.