Now to what could be a medical break through. For five years,f?? we've followed children overwhelmed with crippling fear. 1 in 8 children suffers from anxiety disorders. Tonight, a "20/20" special, new... See More
Now to what could be a medical break through. For five years,f?? we've followed children overwhelmed with crippling fear. 1 in 8 children suffers from anxiety disorders. Tonight, a "20/20" special, new hope, and what we found at the end of the tunnel. Reporter: This whole journey began five years ago, with children consumed with fear. 9 million of them suffer from anxiety in this country. Rocco, who couldn't leave his house, was just 8 years old when his mother took this video of him. Rocco's obsession revolves around the fear of getting sick. The fear about what could happen when he leaves for school. Say, what did you say? I don't remember. Mom. I don't know what I said, honey. Mom. Reporter: His fear is relentless.e1 he wants her to promise him nothing bad is going to happen. Are you sure nothing bad is going to happen? Are you sure nothing -- Rocco, what did I say? Reporter: He begs his mother to repeat herself. Even for the most loving of moms, it can break you. I don't remember what I said. I don't know what I said. Mom. Why do you do that? Reporter: His mother taking this video to show the doctor what they go through every morning together. These images captured just weeks before his diagnosis. Rocco has OCD. His fear of "What if" consumes him. And what about my day? Rocco, I don't -- Reporter: We have followed Rocco for years now. At 12, his parents got him to the carousel at central park. I got on with him, distracting him with talk of little league.t(w3 17-1-1. Reporter: 17-1-1? Doctors can now look at the mris of children who have OCD. They can actually see it. So these are all mris of children with OCD? Exactly right. Reporter: They're convinced it's triggered in part by the chemical glutamate, in the child's brain. In a child who has OCD, you see something in the brain. Either too little or too much glutamate? We see both. There's too little glutamate in certain areas of the brain. There's too much glutamate in other areas of the brain. Reporter: And just this last week, the team revealing they can often pinpoint the type of OCD by looking at the mri. We're in a room full of all different scans. He can point to the one that is most likely Rocco's. So this is Rocco. Yes, that's more likely to be Rocco. Reporter: That's incredible. Armed with this new science, we went back to find Rocco and all of those other children and we were stunned by what we found. Hey, Rocco. David, how you been? Reporter: You're all grown up. Good to see you. You, too. It's like we were here yesterday, right? Reporter: He got me a ticket this time. Still on the journey since that video of him as a little boy. A special "20/20," the children who break away from fear, tonight at 10:00 P.M. Eastern. You can live tweet with the doctors as well.
This transcript has been automatically generated and may not be 100% accurate.