OBJECTIVE: To detect differences in follicular fluid (FF) levels of amphiregulin (AR), depending on mode of triggering final oocyte maturation. DESIGN: Prospective randomized trial. SETTING: Three IVF units. PATIENT(S): Ninety-six patients undergoing IVF-intracytoplasmic sperm injection. INTERVENTION(S): Ovulation triggered with either urinary hCG or GnRH agonist (GnRH-a). Controls: 15 FF samples from small antral follicles (3-9 mm) and 12 FF samples from natural cycle. MAIN OUTCOME MEASURE(S): Follicular fluid concentration of AR, P(4), E(2), vascular endothelial growth factor, and inhibin B. RESULT(S): Significantly lower levels of AR were found in FF from the GnRH-a group versus the hCG group, 51 ± 3.5 versus 71 ± 6.0 ng/mL. In FF from natural cycles, levels of AR were significantly higher than those of GnRH-a triggering but significantly lower than those of urinary hCG triggering. In small antral follicles only 5 out of 15 follicles contained measurable amounts of AR. When urinary hCG and GnRH-a triggering were compared, FF P(4) was significantly higher after urinary hCG triggering, whereas no difference was seen regarding E(2), vascular endothelial growth factor, and inhibin B. A total of 14% more metaphase II oocytes and 11% more transferable embryos were obtained after GnRH-a triggering. CONCLUSION(S): This study suggests that oocyte competence is linked to granulosa cell AR secretion.