OBJECTIVE: To investigate the impact of ovarian stimulation on leptin levels and their effect on in vitro fertilization-embryo transfer (IVF-ET). METHODS: In 39 women who underwent IVF-ET, serum and follicular fluid leptin and E(2) levels were measured by enzyme-linked immunosorbent assay and radioimmunoassay. Blood samples were collected just prior to FSH and HCG injection and follicles puncture. Follicular fluid was collected at the time of oocyte retrieval. RESULTS: The serum leptin levels before HCG administration (26.1+/-2.3)&mgr;g/L were significantly higher than before FSH administration (9.3+/-1.0)&mgr;g/L and oocyte retrieval (15.8+/-2.3)&mgr;g/L(P<0.001). On the day of oocyte retrieval, there was no significant difference between the serum leptin(15.8+/-2.3)&mgr;g/L.and the follicular fluid leptin (18.5+/-2.2)&mgr;g/L. Serum and follicular leptin levels in successful pregnancies were significantly lower(P<0.05) than in women with failed conception. CONCLUSION: It could indicate that high leptin levels may interfere with the developing of dominant follicles.