Ovarian cysts are a common complication of GnRH-a administration. We followed 98 patients who were suppressed with GnRH-a before ovarian stimulation with hMG for IVF treatment. Approximately 20% of the patients receiving GnRH-a during the follicular or luteal phase had developed ovarian cysts. However, the number of cysts per patient was significantly higher in the follicular phase compared with luteal phase. Systematic aspiration of those cysts under local anesthesia permitted the start of ovarian stimulation with hMG as scheduled on day 16 after GnRH-a administration. Follicular fluid content of the cysts revealed similar levels of steroids to those in normal follicles. These cysts contained few cells and no egg. In vitro fertilization treatment was more successful in patients whose cysts were aspirated during the luteal phase than in those with cysts during the follicular phase. We concluded that luteal phase cysts are more benign than follicular phase cysts, and it is possible that they represent an enlargement of pre-existing corpora lutea.