Roberts, who had surgery last month for breast cancer, was to begin chemotherapy Thursday.
Roberts said during Wednesday's broadcast that she would first go into work for the show, which airs from New York City "and then head off for my chemotherapy."
Roberts, who grew up in Pass Christian, Miss., said after the successful surgery her doctors had determined that chemotherapy would be the most effective form of treatment. That will be followed by radiation treatment.
"I'll work as much as I can," Roberts said. "Part of the reason I am coming forward is in case you tune in and it looks like Kojak is sitting next to Diane (Sawyer), you'll understand why."
Roberts thanked viewers, co-workers and guests for their outpouring of well-wishes. She said some viewers even gave tips and ideas for dealing with the treatment.
"Somebody said eat a lot of bacon before I have chemo," she said. "I don't know about that one. That's a new one."
Roberts, who is single, said there is no history in her family of breast cancer.
She's been a contributor to ABC's morning program since 1995 and was the newsreader before being named in May 2005 as one of the show's anchors.