couldn't turn off our cameras. Who can you trust? Here's Elizabeth vargas and David Muir. Bill Cosby, with a growing list of accusers. I spoke to the new accuser just a few hours ago. She echoed what... See More
couldn't turn off our cameras. Who can you trust? Here's Elizabeth vargas and David Muir. Bill Cosby, with a growing list of accusers. I spoke to the new accuser just a few hours ago. She echoed what others said. Tonight, some of the tough questions on both sides. Who will you believe, and who can you trust? Reporter: Bill Cosby is back on TV, but it's not the comeback he was looking for. More and more women accusing the comedian of drugging and sexually assaulting them. And I remember being over there and not having any clothes on. Reporter: You were naked. Tonight in this, her first network television interview, Therese seringese tells "20/20" about her experience with bill Cosby when she was a 19-year-old model visiting las Vegas in 1976. She says in his private dressing room, he handed her some pills. And he said, "Take these." Reporter: Did he tell you what they were? No. Reporter: Did he tell you why? No. Reporter: Did you take the pills? I took the pills. Reporter: What do you remember after the pills? Feeling drugged, and I was kind of leaning forward, and he was behind me having sex with me. And I -- I remember it because it was not good. Reporter: So you didn't go to police? No, you can't tell. Because that's bill Cosby. Nobody will believe you. I was sorry all my life that I wasn't more powerful. I'm sorry I put myself in circumstances where I should have known better. Reporter: It's okay. I know it's very difficult. I'm really sorry. So I'm sorry for my mistakes. But he had no right to do that. Reporter: She is now one of numerous women making similar, disturbing allegations of a sexual attack by bill Cosby. I think what makes these latest developments different is the sheer number. That's one hell of a coincidence if they're all crazy, pathological liars. Reporter: But that's not the bill Cosby most Americans remember. There's no music coming through the headset. You still bopping too much left in your brain? Reporter: He was TV's most trusted funny faced father figure. Pitchman for old fashioned family values, and Jell-O pudding. Warm and wise, and welcome in every living room. He was a groundbreaker. He was a trailblazer. He was a national hero. Reporter: And they trusted him. They absolutely trusted him. Reporter: For decades, nothing shook that trust. Cosby was never convicted or even charged with a crime. But in 2005, it all could have come crashing down. I'm quite certain that this is the closest Cosby ever became to -- to getting arrested. Reporter: One of Cosby's alleged attacks became public for the first time. 31-year-old Andrea constand told police Cosby had drugged and sexually assaulted her in his home near Philadelphia a year earlier. Bruce castor was Montgomery county, Pennsylvania, district attorney at the time. I thought that Cosby had engaged in inappropriate behavior and maybe even criminal behavior. Reporter: But the criminal case was dropped for lack of evidence. Constand then filed settled a lawsuit with Cosby. The details are confidential. But constand's lawyers claimed that 13 women were ready to testify that they too had been attacked by Cosby. We now know one of them was Barbara bowman, who didn't report the alleged assault to police. I was convinced. Who's going to believe me? He's Dr. Huxtable, America's favorite dad. Reporter: This week, Cosby's lawyers called her charges "Decades-old discredited allegations." As for Therese serignese, her account is complicated. And after that first horrible night that you've described, you picked up the phone, and called him, and went back to see him. I did. Reporter: Did you ever have sex with him again? I probably did. Reporter: So everything you did after that first night sounds like not only was it consensual, but you were the one who instigated it. Oh, that's an interesting way to phrase it, isn't that? I would say I never was an active participant in any sexual intercourse with him. I never gave the impression that I wanted sex with him whatsoever. I would say, from my perspective, that I was used. Reporter: So why are these women coming out now? Ironically, the triggering event may have been a fellow comedian. Last month, a video clip posted by Philadelphia magazine went viral. It shows comedian Hannibal Buress taunting Cosby. Cause I had a successful sitcom. Yeah, but you raped women, bill Cosby. So, brings you down a couple notches. Reporter: That unleashed an anti-cosby blizzard on social media. It went viral and everybody suddenly thought, "This guy says to Google, 'bill Cosby rapist.' what happens if I do that?" And they found out. Reporter: The new claims from women that have come forward has escalated far past the point of absurdity. It is long past time for them to stop. Cosby himself has adamantly refused to comment. Just watch what happens when an associated press reporter dares to ask about the claims. I have to ask about when your name coming up in the news. We don't answer that. Can I get an answer from you, that none of that will be shown? Reporter: The days of America looking the other way about allegations against bill Cosby seem to be over. With more women continuing to come forward almost every day, three TV networks have now pulled the plug on Cosby shows. Performances are being cancelled nearly daily. There are many, though, saying that in the first paragraph of that obituary, will be mentioned that he was accused of drugging and assaulting and raping more than a dozen women. This will be a permanent mark on his legacy that will never go away. But, I've seen lesser stars come back from bigger hits. Reporter: Does bill Cosby belong in prison? I'm not about punishment. I've been sorry for 38 years. Now it's time for him to be sorry. We're live on Twitter and Facebook tonight. We want to know what you think. Do you believe the women or Cosby? Or is it more complicated?
This transcript has been automatically generated and may not be 100% accurate.