What percentage of atypical hyperplasia becomes breast cancer?

None: Atypical hyperplasia is not a precancerous condition--it does not turn into cancer in any patient. However, people who have atypical hyperplasia have a 2-4 fold increased lifetime risk for developing breast cancer in either breast, regardless of which breast the atypia was found in.

Small percentage: In a recent study in the annals of surgical oncology, they looked at how many surgical excisions performed for atypical hyperplasia displayed either dcis or cancer. Out of the cases that the study reviewed, only about 1% of the cases upstaged to either dcis or cancer.