Raul Barajas, 27, was last seen about noon Friday at his apartment on West 21st Place.
His family is especially concerned because he still needs to take medication after donating his kidney to his sister.
Laura Barajas says Raul stepped forward to help when she became sick.
"He told me he would do it, that I did not have to wait. He ended up doing it. I knew he was in pain because I was in pain, but he never was upset or you know," Laura Barajas said.
Anyone with information about Raul Barajas should call Chicago police.