And finally tonight our "pern of the week," liching proof you cannot let anyone tell you your future. She was born a child with autism. But tonight or tomorrow night she could become miss america.... See More
And finally tonight our "pern of the week," liching proof you cannot let anyone tell you your future. She was born a child with autism. But tonight or tomorrow night she could become miss america. David muir takes us to meet her, miss montana. I'm so happy. Reporter: Backstage in las vegas, alexis wineman, miss montana, beaming like the others as they compete for the crown. But hers is a journey unlike any other. The little girl with the big smile but hiding underneath it, a giant hurdle. We knew for a long time there was something wrong, we just didn't know what it was. Reporter: The little girl unable to relate to other children at school. The unexplained tantrums. The hours spent alone. She had autism but her parents and teachers didn't know. One teacher saying, I don't get paid enough to handle this. No one talked about it in her small montana town. Diagnosed until she was 11 and nobody thought she would make it here. At 18, alexis defied the odds. The first contestant with autism to compete in miss america. So you've had a little praice? Oh yes. Reporter: And for the talent PORTION SHE PLANs A STAND-UP Act. This is where you have to be funny up there. Yes. I'm always funny up there. Reporter: The same little girl who will tell you she never got the jokes. How did you know you were funny? I use it to deal with situations where I don't understand. Reporter: And now you're going on stage to perform comedy? Yes. It's a pretty ic twist, isn't it? Reporter: The diagnosis came when you were 11 years. You said it was 11 years too late. Because you had wondered all those years why you were different? First impressions with already made. Reporter: But alexis redefined them. Running cross country with her brother. Becoming captain of the cheerleading team. Once so alone and silent she would finish nine years of speech therapy. She was on the high-functioning end of the awe trim spec truck. And when she first began competing in pageants, a borrowed swimsuit from here sister. And old prom dress on the hanger. But this weekend on that las vegas stage no more hand-me-downs. It's all me. I'm really excited about that. Reporter: This is you now. Yes. Reporter: And as we talk we couldn't help but notice she was the only one not up on stage. You're supposed to be rehearsing right now? That's right, I can blame you. Reporter: Her secret to not colliding with the others. Elbows out. Reporter: Quietly in the back of the theater, her mom, who says alexis has won by just getting here. She won. She's here. Sorry. Reporter: You never thought this would even be possible? Absolutely not. This is yont anything -- Reporter: What do you tell her, yourittle girl when she gets on that stages? Be your best. And leave with no regrets. Reporter: What does she tell the others? Hopefully I've touched the others at home. We choose alexis wineman, we'll be watching tomorrow night on abc.
This transcript has been automatically generated and may not be 100% accurate.