Now, to a decades' long mystery that's heating up. Tonight on "20/20," you will hear from an ordinary husband and father, who is, in fact, not quite ordinary at all. He has no idea who he really is.... See More
Now, to a decades' long mystery that's heating up. Tonight on "20/20," you will hear from an ordinary husband and father, who is, in fact, not quite ordinary at all. He has no idea who he really is. It's a remarkable story that barbara walters and her investigative team has been digging into. And barbara here to tell us more about it. You first brought up the story. And it's advanced. I love this story. It's a true mystery. Imagine finding out at 49 years that you aren't who you thought you were. Paul doesn't know the most basic things about himself. His true name. His birthday. Who his real parents are. And he has never opened up and told his whole story. Until now. It was the biggest kidnapping since the lindbergh baby. Paul fronczak, stolen at birth by a fake nurse. The front page was the last anyone had seen of him. Left behind were empty cribs and two broken hearts. Chester and dora fronczak. A year passed. And their phone rang. 800 miles away, in newark, new jersey, a boy had been found. The fbi contacted by parents and said, we think we found your son. My parents drive to new jersey. Reporter: Say this is our baby. We recognize him. Yes. Reporter: Who abandoned the baby had dressed him up, wheeled him to a department store in a new stroller and walked away. Somebody wheeled the stroller. Puts you here. Walks away. And never looks back. Reporter: And never looks back. It's crazy. Reporter: Fronczaks claim the little boy is their own and took him home to chicago. But a year ago, a dna test showed he is not the fronczaks' biological child after all. I'll using his birth certificate. And I want to give it to him. And then, I want to find mine. Reporter: Last week, we returned to newark, new jersey, with paul fronczak. Back to the very place where someone left him sitting in a stroller 48 years before. Is it emotional for you to be here? It is. I feel eerie. I feel anxious. I feel excited. I feel sad. All of those things. Reporter: Paul fronczak, back on the new jersey street, a crossroad in his life. I wish these walls could talk. Maybe we could learn something. I don't know. It's just -- I can't believe this actually happened and i can't believe it's my life. Truly remarkable. Barbara, we're going to learn paul is not the only one on the hunt for answers. When we started to investigate, we found other men who started to say, I'm the real paul fronczak. You'll see who they are. That's remarkable. And paul has a new discovery for himself. We'll learn all about it tonight, barbara. Fascinating stuff. "Stolen at birth." A "20/20" investigation, airs tonight at 10:00, 9:00 central, here on abc. Looking forward to it. Great to see you. Thank you.
This transcript has been automatically generated and may not be 100% accurate.