Roberts, 51, is a breast cancer survivor. She said she is to begin chemotherapy immediately and will receive a bone-marrow transplant with the help of her sister later this year, People.com reported.
"My doctors tell me I'm going to beat this -- and I know it's true," Roberts wrote in a message on the "GMA" Web site.
"If you Google MDS, you may find some scary stuff, including statistics that my doctors insist don't apply to me. They say I'm younger and fitter than most people who confront this disease and will be cured," Roberts said. "I am very fortunate to have a sister who is an excellent match, and this greatly improves my chances for a cure."
Roberts said she hopes to continue working full-time on "GMA" until the transplant, then will take a little time off to recover from the procedure.