Margaret, this is a complicated question because the studies are not conclusive.  Hormone replacement therapy (HRT), which combines the estrogen with the drug in Provera has definitely been linked to increased breast cancer risk, but that is more likely to be from the estrogen part of the HRT than the Provera part.  I don't think there is a study on the Provera alone that would show increased risk, but your doctor will be more up to date on that.  As with all health questions, you have to balance the potential risk of breast cancer with your current endometrial issues.  Ask your doctor more questions about possible breast cancer risks, and don't hesitate to ask for a second opinion from another doctor if you are still not sure.  Only a doctor who can see your entire medical history and who knows how potentially dangerous your current problem is can help you weigh the risks and benefits for you.