Francis Cardinal George began chemotherapy Wednesday to treat cancer in his liver and right kidney.
The Archdiocese of Chicago said the Cardinal, who previously survived bladder cancer, will undergo six sessions of chemo with each session running three weeks (two weeks of treatment, one week of recuperation).
Doctors said last month they found a nodule on the 75-year-old Catholic leader's liver that contained cancerous cells, along with cancer in his right kidney.
The Archdiocese said Cardinal George plans to maintain a regular work schedule during the four-month course of treatment, but his public schedule will be reduced by half during the weeks he's not receiving chemo.
Doctors will share their analysis of his condition after the treatment is completed.
"With a grateful heart, Cardinal George would like to acknowledge all the people who have sent cards and email notes expressing their concern and promising their prayers," the Archdiocese said in a statement. "Please continue to keep the Cardinal in your thoughts and prayers."
The Cardinal submitted a letter of resignation to Pope Benedict XV this year, a formality when Catholic bishops turn 75, but it wasn't accepted. He is expected to remain the head of the Chicago Archdiocese for two or three years.