Four different major clinical complications were identified in a retrospective analysis of 2495 in-vitro fertilization (IVF) cycles resulting in oocyte retrieval. The severe form of ovarian hyperstimulation syndrome (OHSS) occurred in 18 patients, giving a prevalence for this complication of 0.7%. Seven (39%) of these 18 patients had previously been diagnosed as having polycystic ovaries. Eleven patients were admitted with moderate OHSS. Adnexal torsion was diagnosed in two patients. Ovariectomy was considered necessary in both cases. Complications of the transvaginal procedure occurred in seven cases (0.3%): one patient had an acute appendicitis with puncture holes in the appendix, six patients were admitted shortly after oocyte retrieval with a pelvic inflammatory disease. Of the 624 pregnancies obtained, 13 were ectopic, giving an ectopic pregnancy rate of 2.1%. It is concluded that serious clinical complications of IVF treatment are rare. However, patients should be counselled for the occurrence of serious procedure-related complications before entering an IVF programme.