"This is how I was born. I mean, there's no doubt in my mind," Chaz told "Nightline" anchor Cynthia McFadden.
In a rare and candid interview at his home in West Hollywood, Calif., Bono, 42, openly talked about going under the knife and going through a process known as "transitioning" -- changing physically from female to male. This life-altering decision is described in his latest memoir, "Transition: The Story of How I Became a Man," and in his documentary, "Becoming Chaz."
"It's actually pretty simple if you look at it," Bono said. "We all in the womb start out as female and then hormones come and we either stay female or we become male. I think of it as hormones that, you know, went in the brain but not in the body, and that's all being transgender is. It's just that the sex of your body and the gender of the brain don't match up."
It took Bono nearly four decades to reach the point at which he decided he wanted to have gender reassignment surgery to become a male. The process began after Bono's 40th birthday in March 2009 -- over a decade after he had publically come out as a lesbian.
Bono began receiving testosterone injections, which helped make his voice deeper and produced facial hair. He also had his breasts surgically removed.
"If you are a man and you have breasts, any man would want to have them removed," Bono said. "It is scary for a woman to think about it because it is something that they are really attached to. Being male and having breasts is about the worst thing I could imagine."
But the process also had an incredible emotional toll attached to it, not just for Bono, but also for his family, including his mother, Cher, and his partner, Jennifer Elia.
"If I was average Joe I would have done this years ago," Bono said. "I felt a tremendous responsibility not just for myself, but what am I doing to my family? What am I going to put them through?"
In an exclusive interview with "Nightline" anchor Cynthia McFadden last year, Cher talked about how difficult it was for her when her daughter first came out as a lesbian, and then told her she wanted to become a man.
Elia, Bono's fiancée, said she was also very nervous about her partner having the surgery and how the public would react to his new transformation afterwards.
"I was terrified primarily of what other people would say and think," Elia told McFadden. "I thought people were going to throw dog feces at our door."
Bono shared his partner's sentiment: "I was worried about being followed by the tabloids again. I risked a lot but I finally got to the place where I knew this is what I'm supposed to do."
Elia and Bono met in 2005 and hit it off immediately. Just a few months into their relationship, Elia heard the bombshell. Chaz was thinking of changing his gender, and then he decided to go ahead with it.
"I was like, 'You're going to go do this, OK, what about me? What about everyone in your life? We're just supposed to adjust?'" Elia said. "I didn't go through it sober."