The aim of the present study was to determine the effect of octreotide, a somatostatin analogue, on ovarian sensitivity for follicle stimulating hormone (FSH) in patients with polycystic ovary syndrome (PCOS). As the measure of ovarian sensitivity, the FSH threshold was determined in a case-control set-up. Eleven patients with PCOS were treated with FSH in a low dose step-up manner and subsequently received treatment with FSH combined with octreotide. The FSH threshold was found to be significantly higher during combined treatment. This increase was associated with a decrease in insulin-like growth factor-I (IGF-I) and IGF binding protein-3 (IGFBP-3) concentrations during treatment with octreotide, but not with a decrease in insulin concentrations. No differences were found between the two regimens, in number of follicles, in oestradiol concentrations on the day of human chorionic gonadotrophin (HCG) administration or in ovulation rate. With both treatments, there was a very low rate of multifollicular development. It can be concluded that octreotide lowers ovarian sensitivity for FSH through suppression of IGF-I/IGFBP-3 in patients with PCOS. However, this does not appear to affect follicular development during gonadotrophin stimulation, because the latter is controlled to a high degree by the use of a low dose step-up treatment schedule in these patients.