Breast cancer patients are commonly treated with drugs that eggs present in the ovary and may reduce their chance for getting pregnant. Their fertility can be preserved by stimulating their ovaries, collecting multiple eggs, fertilize them in the lab and freeze them. Ovarian stimulation increase their estrogen levels in blood.this may stimulate their cancer and increase chance for recurrence. If a medicine that prevent estrogen rise is used (letrozole), this may increase the safety of stimulation. In this study we compared ovarian stimulation in breast cancer patients using letrozole with those who did not undergo stimulation and showed that there is no increase risk for recurrence after a median follow up of 2 years