FBI Re-Opening 1964 Kidnapping Case of Baby Boy Taken From Chicago Hospital.

Paul Fronczak learns decades later he was reunited with the wrong family.
3:00 | 08/08/13

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Transcript for FBI Re-Opening 1964 Kidnapping Case of Baby Boy Taken From Chicago Hospital.
Tonight the fbi is asking for everyone watching this program to help solve a mystery. Re-opened today after nearly 50 years, a new born was kidnapped from his mother in a chicago hospital, later returned or so the world believes. Now after months of investigation by abc news and "20/20's" barbara walters, you hear exclusively from that little boy all grown up and he is a man who has learned a stunning secret about his life. Reporter: Looking at old family photos with paul fronczak is a strange experience. Because the baby in his baby pictures is not him. How do you feel when you see this picture? I feel like I want to find him, and hug him, and make sure he's ok. Reporter: Paul fronczak recently discovered his entire life -- his name, his parents, his birthday, all of it, is a big mystery. A mystery that began with a kidnapping 49 years ago, in 1964, just a day after paul fronczak was born, the first child of chester and dora fronczak. But baby paul was snatched from his mother's arms right in the maternity ward of a chicago hospital. The baby was kidnapped? The baby was gone. I don't know how someone can come and take a woman's baby. Reporter: Dora fronczak's public appeal to the kidnapper went unheaded -- do you have any suspects here in chicago that you are going to question? We are questioning suspects throughout the day. Reporter: A mass tif dragnet came up empty. Just over a year later a child was thought to be him and the parents took him home. Are now an adult, paul works for a college in nevada. He's married around has a child of his own, a normal life, until a year ago when a dna test showed his not the fron zach's child after all. What are you trying to accomplish? My main goal is to find the real paul. My parents raised me and did a great job. I feel that if I don't do everything I can to help find their real child, then I'm not doing my job as a son. This is an incredible mystery. Barbara walters is here. Now that the fbi has re-opened the case does he really think it can happen? Let's say he's cautiously optimistic. He hopes he may have better luck from the general public than he might get from officials. 20/20" is putting the full resources of the abc news investigative team into the effort to help him solve the two greatest mysteries of his life, who he is and whatever happened to the real paul fran zach. If you know anything about this case, let us hear from you. There are answers out there somewhere. If you have tips please go to our website, abcnews.Com slash "20/20."

This transcript has been automatically generated and may not be 100% accurate.

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