The aim of this study was to investigate the late follicular phase of seven gonadotrophin-treated patients with polycystic ovary syndrome (PCOS) exhibiting monofollicular growth and to compare developmental characteristics with the dominant follicle in seven regularly cycling control women. Daily serum follicle-stimulating hormone (FSH) levels in patients with PCOS decreased more rapidly compared to controls (−0.3±0.2 IU/day in controls versus −0.7±0.4 IU/day in PCOS; p<0.02). No statistically significant differences were seen in daily increase (30% in controls and PCOS) and mean peak levels (825±94 pmol/l in controls versus 937±231 pmol/l in PCOS) of oestradiol (E2) serum levels when comparing both groups. Mean daily growth of the dominant follicle (1.7±0.4 mm in controls versus 1.9±0.6 mm in PCOS) was not significantly different. It is concluded from the present study that in patients with PCOS treated with gonadotrophin plus adjuvant gonadotrophin-releasing hormone agonist, development of a single follicle can occur using a decreasing dose regimen resulting in decreasing serum FSH levels. In addition, growth and oestrogen production by the dominant follicle in PCOS is not significantly different from follicle growth under normal conditions.