"The truth is I carry a 'faulty' gene, BRCA1, which sharply increases my risk of developing breast cancer and ovarian cancer," Jolie, the mother of six young children, wrote. "Once I knew that this was my reality, I decided to be proactive and to minimize the risk as much I could. I made a decision to have a preventive double mastectomy."
Jolie's mother, Marcheline Bertrand, died of ovarian cancer in 2007 when she was 56.
Jolie, 37, said she had her breasts removed and reconstructed over the last few months in an attempt to lower her chances of getting cancer.
"I am fortunate to have a partner, Brad Pitt, who is so loving and supportive," Jolie wrote. "Brad was at the Pink Lotus Breast Center, where I was treated, for every minute of the surgeries. We managed to find moments to laugh together. We knew this was the right thing to do for our family and that it would bring us closer. And it has."
Jolie said she wanted to write about what she went through "because I hope that other women can benefit from my experience."