This pilot study evaluated the effects of coadministration of metformin with clomiphene citrate (CC) and human menopausal gonadotrophin (HMG) in women with CC-resistant polycystic ovary syndrome (PCOS). Sixty women with PCOS were randomly assigned to receive 3 months' treatment with metformin or placebo together with CC and HMG. Transvaginal ultrasound was used to monitor follicular development and ovulation was induced by human chorionic gonadotrophin (HCG). The number of dominant follicles, the oestradiol level on the day HCG was given and the amount of HMG required were significantly lower in the metformin group than in the placebo group, whereas the mono-ovulatory rate and pregnancy rate in the third cycle were significantly higher. The cumulative pregnancy rate in the metformin group (43.3%) was higher than in the placebo group (20.0%), but this difference did not reach statistical significance. In conclusion, coadministration of metformin with CC and HMG reduced the amount of HMG required and increased the mono-ovulatory rate and pregnancy rate.