The objective of this study was to investigate whether the incidence of monofollicular growth during stimulation with low dose follicle stimulating hormone (FSH) changes when adjuvant gonadotrophin-releasing hormone agonist (GnRHa) pre-treatment is administered in polycystic ovary syndrome (PCOS). One group of patients (group 1) suffering from clomiphene resistant PCOS was stimulated with low dose FSH. The results were compared with those from another group of similar patients (group 2) subsequently stimulated with low dose FSH combined with GnRHa. In group 1 15 patients had 39 stimulation cycles performed; in group 2 13 patients had 33 stimulation cycles performed. In group 1 44% of cycles were monofollicular, whilst the corresponding figure in group 2 was 14% (P = 0.04). Evidence was found for postponed atresia in group 2. In both groups 1 and 2 inter-individual and intra-individual variability of the FSH dose inducing follicular growth were observed. We concluded that during the use of GnRHa, stimulation with low dose FSH less frequently resulted in monofollicular growth, possibly due to postponed atresia. Furthermore, the use of GnRHa does not abolish the inter- and intra-individual variability of the FSH dose inducing ongoing follicular growth.