Roberts, 51, who expects to undergo a bone marrow transplant this fall with her sister as a donor, learned of the diagnosis on the same day that GMA beat NBC's "Today" show in viewer ratings for the first time in 16 years, she said in a statement on the network's website.

"Talk about your highs and lows!" Roberts said.

"My doctors tell me I'm going to beat this - and I know it's true," she said in the statement.

The network's medical correspondent, Dr Richard Besser, said in a statement on the website that he was consulting with Roberts about MDS, a rare malignant disorder of the bone marrow that typically affects elderly people and can sometimes be the result of cancer treatment.

He said her treatment would begin on Monday, when she receives a drug to prepare her for the bone marrow transplant. She will continue hosting the morning television show but is expected to stay off the air for several months after the transplant to recover.

(Reporting by Barbara Goldberg; Editing by Eric Beech)

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