“Good Morning America” co-anchor Robin Roberts told viewers Monday she has been diagnosed with a blood disorder likely caused by her treatment for breast cancer five years ago, and will undergo a bone marrow transplant later this year.

“Sometimes treatment for cancer can lead to other serious medical issues, and that’s what I’m facing right now,” she said near the end of Monday’s “GMA.”

Roberts explained her illness is called myelodysplastic syndrome (MDS) and is a rare blood disorder that affects the bone marrow.

On ABC News’s Web site, Roberts wrote that she learned about her MDS diagnosis on the day that “Good Morning America” beat NBC’s “Today” show for the first time in 16 years, adding, “talk about your highs and lows!” The same day that she was having “a rather unpleasant procedure to extract bone marrow for testing,” she added, she learned she would conduct the next day her much-picked-over interview with President Obama about his views on same-sex marriage.

“The reason I’m sharing this with everybody now is because later today I begin what’s known as pre-treatment,” Roberts said near the end of Monday’s “GMA” broadcast, surrounded by her co-anchor, George Stephanopoulos, and other show regulars.

In her Web site message, Roberts said she begins chemotherapy in advance of a bone marrow transplant. Her older sister, a “virtually perfect match,” will be her bone marrow donor, she said.

Robin Roberts on “Good Morning America,” with Josh Elliott, George Stephanopoulos, Lara Spencer and Sam Champion. (Ida Mae Astute/ABC)