Cardinal Francis George presided over the blessing of the palms before mass.
Last week he announced he would participate in holy week activities, even though he is currently undergoing a second round of chemotherapy for cancer in his kidney.
Parishioners say Sunday's mass was very special.
"Their leader, the shepherd of the flock, who didn't have to be here today, but chose to be here to be with his people," said parishioner Joe Sotolar.
"It was very touching and I really enjoyed having the opportunity of being here today and I'm glad that I came here," said parishioner Marina Coser.