Bishop R. Daniel Conlon of Joliet announced he is taking a medical leave of absence starting Dec. 27.
Retired Bishop Richard E. Pates of Des Moines, Iowa, will be apostolic administrator of the diocese while Bishop Conlon is away.
A statement from the diocese did not disclose the exact reason for the medical leave.
Bishop Conlon expressed in the statement "his deep affection" for the clergy, religious and laity of Joliet and said he planned to keep them in is prayers during the time away from his responsibilities.
He also asked for prayers from the faithful of the diocese.
Bishop Pates said he welcomed "the opportunity to journey with the priests, deacons, religious and laity of the local church."
"The diocesan community enjoys an outstanding reputation, and I feel privileged to participate in your faith life," Bishop Pates said in a statement.
He called Bishop Conlon "a good bishop" who will "benefit from his time away."
"We, the Catholic Church, dedicate ourselves anew to the life-giving message of Jesus Christ as we embrace a new year and a new decade," Bishop Pates added. "May we humbly experience 'the Joy of the Gospel' in the spirit of our beloved Holy Father, Pope Francis."