Transcript for Robin Roberts' emotional health journey to milestone 'birthday'
Reporter: Robin Roberts has a lot to celebrate. It's a birthday party. Marking five years since her life-saving bone marrow transplant. Your beloved mother said, as you often quoted, make your mess your message. What's your message today? Left foot, right foot, breathe, keep moving, keep believing. That the best is yet to come. I pray that I am a symbol of resilience. That of hope. Reporter: It's that hope which propelled her on the long road to healing. Robin shared some news with us -- Reporter: She shared with the world back in 2012, just a few years after she overcame breast cancer. Sometimes treatment for cancer can lead to other serious medical issues. And that's what I'm facing right now. It is something that is called mds. Reporter: Myelodysplastic syndrome, mds, a rare blood disorder that required an agonizing bone marrow transplant. My big sister is a virtually perfect match for me. And she is going to be my donor. She's going to be my donor. Reporter: Robin's the youngest of four. All three of her siblings volunteered to be her bone marrow donor, but only Sally Ann was a match. It wasn't until we were matched you told me only 30% of people looking for a match find it in their family. What was that moment like when you discovered you were a match? I was ecstatic. It was -- it was an answer to prayer. Rejoice. Rejoice that you have been chosen to help someone. Reporter: Five years ago today, surrounded by her family and close friends, she was given the gift of life. ??? I feel all the love in here. All the love. We're keeping it boring. Keep it boring. Reporter: This is the moment her doctor inserted millions of Sally Ann's stem-cells into a port in robin's chest. I say a small prayer, a small saying that both my wife and mother-in-law have. Let god do miss work, and it will work. I thought, this is why I have this man by my side. This is why we have been brought together. I think that was a good in my opinion to say, go Sally go! Reporter: The weeks after robin's transplant were brutal, spent mostly in isolation. If she wanted to leave her room, she had to wear mask and gloves as did everybody around her, as she built up her immune system. We got to stop meeting like this. ??? Rejoice rejoice ??? ??? Emanuel ??? ??? we love you dear sister yes we do ??? ??? rejoice oh robin ??? We have to show people the difficulty, the challenges. I'm not going to sugarcoat it. It's only when you go through the darkness that the light is so bright. Going home now! Reporter: After 30 days she walked out of the hospital and took her first breath of fresh air. Robin's story a powerful inspiration. Thousands registered with the simple swab to be the match. The national marrow donor program. Our story led to 18,000 people joining the registry of be the match. Of those 18,000, 173 were a match. And they gave me this pin, meaning the potential of 173 lives were saved. Reporter: Among those lives saved, A.J., who was diagnosed with leukemia on his 4th birthday. And by the summer of 2015, he needed a bone marrow transplant. The first step was for all of us to be tested. None of us were a match. Reporter: The family turned to be the match, and within weeks they found one. A 22-year-old female. That's all the family knew. A.j.'s mom wrote her a letter. I thank you from the very depths of my heart. For what you've done for my little boy. Reporter: After 270 days in the hospital, the transplant worked. And now, 2 1/2 years later, 8-year-old A.J. Is cancer-free and back in school. And this morning, he's about to meet his donor for the first time ever. Face-to-face. Saying thank you, that never really feels like enough when someone saves your child's life. A.j.'s match is here, though you've never met in person. You've talked on the phone but haven't met in person. Are you ready to meet her? Yeah, really ready. My heart is pounding. I don't know if I can take this. All right, everybody, give a round of applause to a.j.'s match. Alex monley. She didn't know A.J. She didn't know who was going to receive that. And to be so unselfish. And that's just what I thought. This beautiful, unselfish young woman. Something you want to say to her? Thanks for saving my life. It was my pleasure. Why did you want to be a donor? Because it's the right thing to do. It's so beautiful. I know that without even meeting him, you wrote a letter. I did. Can you read on portion of it? This boy may be somebody's -- may someday be someone's husband, someone's father, grandfather, son-in-law. Maybe he will take the world by storm and find the cure for cancer, or maybe he won't. The point is, he is important. If everyone could join the Reporter: Robin's life was saved, yet there are still challenges. How is your immune system? Early on you talked about having your immune system of a newborn, of a 2-year-old, of a 5-year-old -- Okay, let's be honest. I'm doing very, very well. I've had some setbacks. There have been hospitalizations. There's been medication. But I can look you in your beautiful eyes and tell you, I am blessed. I am doing well. There's no detection of any sort of virus or illness. I feel stronger every day. It took some time to get there. Reporter: Robin's is a story of survival, of rebirth. But it's also a story of sister ly love. The one thing that came out of this for me is a greater appreciation for you. A greater appreciation for your strength and your ability. And the fact that you're here. That the thought of possibly losing you has made me appreciate every phone call, every time we're together. And I'm just so very grateful that you are here. Reporter: For "Nightline" I'm juju Chang in New York.
This transcript has been automatically generated and may not be 100% accurate.