OBJECTIVE: The key enzyme in the biosynthesis of estradiol, aromatase, has been demonstrated within endometriosis. Combined administration of aromatase inhibitor and GnRH-agonist may efficiently suppress estrogen biosynthesis through a combined pituitary, ovarian, peripheral and "in situ" action. The aim of this study was to test the concept of combined down-regulation prior to IVF in patients with endometriomas. STUDY DESIGN: Prospective pilot study in a university-based tertiary fertility clinic including 20 infertile patients with endometriomas undergoing IVF/ICSI. The patients received goserelin 3.6mg sc on treatment Days 1, 28 and 56, and one daily tablet of anastrozole 1mg from Day 1 to Day 69. Controlled ovarian stimulation was initiated from Day 70. Outcome measures were change in endometriomal volume and serum CA125 during down-regulation, standard IVF parameters including pregnancy and delivery rate, and endocrine response. The paired T test and Wilcoxon Signed Rank test were used to analyse paired differences. RESULTS: During the combined down-regulation, the endometriomal volume and the serum CA125 level decreased by 29% (3-39%) and 61% (21-74%), respectively (median (95%CI), P=0.007 and P=0.001). In the IVF/ICSI cycle, the number of oocytes retrieved was 7.5 (6.0-10.0) and the fertilization rate was 0.78 (0.38-1.0). Nine patients (45%) conceived, five (25%) had a clinical pregnancy, and three (15%) delivered healthy children (two singletons and one twin). CONCLUSIONS: Prolonged combined anastrozole and goserelin down-regulation significantly reduces endometriomal volume and serum CA125, and is compatible with IVF pregnancy and delivery. However, a high pregnancy loss was noted.