"I was hoping to be able to wait until tomorrow, but this morning will now be my last show for awhile before I begin my treatment next week," Roberts announced on Thursday's show.
Roberts said she had to leave a day earlier to visit her ailing mother in Mississippi.
In June, Roberts revealed that she had myelodysplastic syndrome, or MDS, from complications from the breast cancer treatment she underwent five years ago. Roberts will get a bone marrow transplant from her sister.
While she is on medical leave, several people from ABC will be filling in for Roberts, including Diane Sawyer, Katie Couric, Barbara Walters and Elizabeth Vargas. Oprah Winfrey will also be filling in, along with some surprise guests.
It is not yet known when Roberts will return to the morning show, but she said she will return "as soon as I can."