Fourteen ovulatory patients undergoing diagnostic laparoscopy had at least 2 samples of clear follicular fluid (FF) collected in the late follicular phase. Dominant follicles contained significantly higher concentrations of insulin-like growth factor-1 (IGF1), oestradiol (E2), progesterone (P) and lower androstenedione (AD) than their matched cohorts. After the luteinizing hormone (LH) surge, a significant increase in dominant FF-IGF1 and FF-P occurred. Log FF-E2 and log FF-P were correlated with both FF-IGF1 and FF volume. Log FF-AD was negatively correlated with FF-volume, FF-E2 and FF-P (but not FF-IGF1). The cohort concentration of FF-IGF1 was correlated with serum IGF1. These results support the hypothesis that IGF1 has a central role in the selection of the dominant follicle.