Since gonadotropin-releasing hormone (GnRH) analogs were introduced into clinical therapeutic use, several side effects directly related to the hypoestrogenic state have been reported. The authors have encountered a rather infrequent complication, namely ovarian cystic formations, when using these compounds for selected in vitro fertilization and embryo transfer (IVF-ET) cases. In 7 of 24 patients with Decapeptyl (D-Trp6-luteinizing hormone-releasing hormone [LH-RH], Ferring, Kiel, FRG) treatment, and in 5 of 22 patients treated with Buserelin (Superfact, Hoechst A.G., Frankfurt, FRG), solitary ovarian cysts developed during the down-regulation phase. Their growth did not change during ovulation induction with menotropins. Although the mechanism of ovarian cyst formation during GnRH agonist treatment is not clear, their presence does not appear to interfere with the fertility of these women.