The incidence of ovarian cyst formation during stimulation with additional pituitary suppression was retrospectively studied in 359 patients included in our in-vitro fertilization (IVF) programme. Women were classified according to the type of pituitary desensitization with subcutaneous buserelin used in group A (long protocol; n = 285) and group B (short protocol; n = 74). The rate of appearance of single follicular ovarian cysts for group A was 9.82% and for group B 22.97% (P less than 0.005). Ovarian cystic formations were usually asymptomatic and nonfunctional. The presence of these cysts did not seem to interfere with the ovarian response to stimulation treatment. Oocyte retrieval and pregnancy rate were similar between patients who developed ovarian cysts during gonadotrophin-releasing hormone analogue (GnRHa) therapy and those without cyst formation. These results suggest that ovarian cysts developing during GnRHa treatment are probably the consequence of the initial gonadotrophin rise and that the presence of ovarian cysts in these conditions should not be considered a necessary cause of cancellation for IVF patients.