Among 30 patients with polycystic ovary syndrome, treated with low-dose gonadotrophins, 75 cycles were analysed in order to characterize overstimulated cycles that were at increased risk of developing ovarian hyperstimulation. Optimal response (one or two follicles > or = 14 mm diameter) was observed in 59 cycles (79%). The remaining 16 cycles (21%) exhibited an overstimulated response characterized either by growing more than two follicles or having an oestradiol level > 850 pg/ml (2 SD above the mean observed in optimal cycles). Six of the latter were handled prospectively when oestradiol levels were found to be too high according to the size of the leading follicle. This stage was termed as developing overstimulation and its identification was based on objective criteria obtained from the optimal group. Following the withholding of gonadotrophin, the follicles continued to grow; however, the final oestradiol level was lower compared with six other matched overstimulated cycles. Overall, 14 patients conceived (47%) of whom three (21%) had multiple pregnancies. Mild or moderate ovarian hyperstimulation syndrome occurred in three cases; all of which involved overstimulated cycles. Low-dose gonadotrophin treatment is associated with a substantial degree of overstimulated response. All cycles should be monitored carefully in order to recognize the overstimulated response, which deserves cautious management.