Angelina Jolie has undergone a preventive double mastectomy because she had a very high risk of breast cancer.
Jolie said she was speaking out to encourage other women with a family history of cancer - her mother died of it age 56 - to seek out information from doctors and make informed choices about their bodies.
She said she managed to keep the issue quiet and keep working.
"But I am writing about it now because I hope that other women can benefit from my experience," she said.
The 37-year-old American actress wrote in an opinion piece entitled "My Medical Choice" in The New York Times that she had chosen the procedure because she carries a faulty gene that increases her risk of both breast and ovarian cancer.
Jolie, one of Hollywood's best-known faces and the partner of actor Brad Pitt, said that because of this gene, known as BRCA1, her doctors estimated she had an 87 per cent risk of breast cancer and 50 per cent risk of ovarian cancer.
"Once I knew that this was my reality, I decided to be proactive and to minimise the risk as much as I could. I made a decision to have a preventive double mastectomy," she wrote.
"I started with the breasts, as my risk of breast cancer is higher than my risk of ovarian cancer, and the surgery is more complex," Jolie wrote.
She said that on April 27 she completed the three months of medical procedures that the mastectomies involved.
Jolie said her chances of developing breast cancer are now down to five per cent.
Jolie and Pitt have three adopted and three biological children.
"I can tell my children they don't need to fear they will lose me to breast cancer," Jolie said.
Jolie described a two-stage surgical process, the second of which is an operation that can take up to eight hours as the breast tissue is removed and temporary fillers are put in place.
"You wake up with drain tubes and expanders in your breasts. It does feel like a scene out of a science fiction film. But days after surgery you can be back to a normal life," Jolie wrote.
Jolie said Pitt has been a huge source of support - "we managed to find moments to laugh together," she said - and she has only small scars after the ordeal, with nothing alarming for her children to see.
"On a personal note, I do not feel any less of a woman. I feel empowered that I made a strong choice that in no way diminishes my femininity."
Jolie, one of the world's highest paid performers, said the cost of getting tested for BRCA1 and another faulty gene, called BRCA2, is more than $US3000 ($A3025) in the United States and that this "remains an obstacle for many women".
She said she hopes women living under the threat of cancer will be able to get tested.
"Life comes with many challenges. The ones that should not scare us are the ones we can take on and take control of," Jolie wrote.