Objective: To assess the effects of leptin on follicle-stimulating hormone (FSH) and insulin-like growth factor-I (IGF-I) induced production of estradiol (E(2))-17beta and progesterone (P) in cultured human luteinized granulosa cell (GC)., Methods: Human luteinized GCs were obtained from pre-ovulatory follicles in an in-vitro fertilization program, and were cultured for 24 hours in the presence of leptin (3.0 ng/ml) or FSH (1.0 ng/ml) or IGF-I (30.0 ng/ml) alone, leptin with FSH or IGF-I or both, and FSH with IGF-I. The conditioned media were aspirated for measuring E(2) and P content. Luteinized GC were also counted for cell number and detected the expression of the leptin receptor by reverse transcription polymerase chain reaction analysis., Results: In cultured luteinized GC system, leptin or FSH or IGF-I alone did not affect the growth of cultured human luteinized GC. Leptin alone or in the presence of FSH had no effect on E(2) production. E(2) levels in culture media without leptin or FSH were (0.103 +/- 0.036) pmol/1 000 cells, (0.323 +/- 0.042) pmol/1 000 cells respectively. When cultured with leptin alone or with leptin and FSH, E(2) levels were (0.120 +/- 0.008) pmol/1 000 cells, (0.343 +/- 0.034) pmol/1 000 cells. Leptin significantly inhibited FSH + IGF-I or IGF-I induced E(2) production. E(2) levels decreased from (0.318 +/- 0.037) pmol/1 000 cells to (0.193 +/- 0.025) pmol/1 000 cells (IGF-I) (P < 0.05) and from (0.493 +/- 0.036) pmol/1 000 cells to (0.251 +/- 0.033) pmol/1 000 cells (FSH + IGF-I) (P < 0.01). However, the production of P did not change. The leptin receptor expression was demonstrated in luteinized GC., Conclusions: Leptin may directly attenuate the IGF-I or IGF-I and FSH-stimulated E(2) synthesis in cultured human luteinized GC by the leptin receptor mechanism.