The purpose of this study was to assess the impact of 3 months of aromatase inhibition together with progestin add-back on ovarian endometrioma size. This prospective cohort study was performed at University Medical Center (UC San Diego). Women trying to conceive were excluded. After informed consent, all women were treated with the aromatase inhibitor letrozole (5 mg/d) with norethindrone acetate (5 mg/d) add-back for 3 months. Pre- and posttreatment assessments of endometrioma sizes were performed by ultrasound. The impact of treatment on pain was determined using the patient assessed endpoints of the Biberoglu and Behrman scale. These included assessing dysmenorrhea, dyspareunia, and nonmenstrual pelvic pain each on a scale from 0 to 3. The primary endpoint of this study was the change in ultrasound documented endometrioma size. Fourteen endometriomas in 8 consecutive women were treated for 3 m. Mean endometrioma diameter decreased 50% from 4.6 ± 1.6 cm to 2.3 ± 1.6 cm (mean ± SD). This represents a 75% decrease in endometrioma volume. Endometriosis symptoms of dysmenorrhea, dyspareunia, and nonmenstrual pelvic pain also improved with treatment. In conclusion, a 3-month course of high dose aromatase inhibition with progestin add-back significantly reduces ovarian endometrioma size and warrants further investigation.