(Screenshot via People)

Gloria Vanderbilt dropped a bombshell on Anderson Cooper when she revealed she once had a lesbian affair.

During an interview with People, Vanderbilt told Editorial Director Jess Cage she had once been involved in a same sex relationship at 13 years old.

“What? Hello, this is news to me. You didn’t mention this in the book, mom,” Cooper says.

‘I went through a brief so-called lesbian relationship with a girl in school,’ Vanderbilt, 92, told People. “Cynthia, her name was, and she came once to visit my aunt in New York on holiday. We had this sort of lesbian relationship and it felt so great. It felt so good and yet I thought, ‘There’s something about this.'”

Cooper, 48, says Vanderbilt’s mother Gloria Morgan Vanderbilt was accused of being a lesbian during the 1932 custody trial over Vanderbilt.

“That word wasn’t spoken. The press wasn’t supposed to write about it,” Cooper says. “My mom knew that something terrible had come out in the court, but she didn’t know what it was and she didn’t know what being a lesbian meant at the time, obviously.”

Vanderbilt says the affair was before the trial, but she was still wary about her feelings.

“This is before the thing I knew about my mother,’ Vanderbilt says. ‘I thought, “No, this is something that’s not really what I want.” It was very brief.’

“I think almost everybody goes through at one point. Of course, the thing is, now we realize there’s no difference. Love is love,” Vanderbilt added.

Cooper also spoke about how his mother’s attitude about same-sex relationships encouraged him to come out to her. As a child, Cooper says his mother was friends with a gay couple who she described to him as married even though same-sex marriage was illegal at the time.

Cooper and Vanderbilt have written “The Rainbow Comes and Goes: A Mother and Son Talk About Life, Love and Loss,” a joint memoir together out in stores April 5. They will also star in the HBO documentary about their relationship “Nothing Left Unsaid” airing April 9.