It's called myelodysplastic syndrome, or MDS. It's a blood and bone marrow disease, linked to the cancer treatment that once saved her life. The 51-year-old broke the news on Monday morning's show with her co-hosts by her side.
"Sometimes, treatment for cancer can lead to other serious medical issues. And that's what I'm facing right now," she said.
About 18,000 cases are diagnosed each year. Only a few hundred are caused by chemo or radiation. Sometimes cancer treatment can affect the DNA of blood-producing stem cells. Later, the fear is that it can turn into leukemia.
Roberts begins pre-treatment chemotherapy to prepare for a bone marrow transplant later this year. Unlike other minorities who can't find a marrow match due to the low number of registered donors, Roberts' older sister turned out to be a perfect one.
For Roberts, the past two months have featured some major highs and lows. This weekend, she was inducted into the Women's Basketball Hall of Fame. In April, "Good Morning America" beat the "Today" show for the first time in 16 years. But on the day that should have been a career highlight, Roberts received her MDS diagnosis.
During a painful procedure to extract bone marrow for testing, Roberts found out she would sit down with President Barack Obama the very next day for a groundbreaking interview where the president announced his support for same-sex marriage.
Roberts said it's all about focusing on the fight, not the fright.
"I'm like everyone who faces some life-altering situation, whether it's your health or finances or what not. And it's getting up off the mat and fighting," she said.
After Michelle Obama learned about Roberts' new health battle, the first lady tweeted, "Barack and I have you in our prayers. We believe in you and thank you for bringing awareness and hope to others."
Roberts said while she will miss occasional days for treatment, she will remain on the show. She will be taking extended time off when she has the actual treatment.
Symptoms of MDS include fatigue and anemia. ABC senior health editor Dr. Richard Besser says it's very rare, and it's not a reason for cancer patients to forgo their treatment.
Roberts hopes her bone marrow transplant will inspire others to register to become marrow donors.