This study examined the effect of 17β-estradiol (E<sub>2</sub>) on basal and luteinizing hormone (LH)-releasing hormone (LHRH)-stimulated gonadotropin secretion in 9 patients with Klinefelter’s syndrome. Intramuscular injection of E<sub>2</sub> (10 µg/kg/day during 5 days) induced a rapid decrease in follicle-stimulating hormone (FSH) and LH levels. The maximum suppression was observed on day 7 (D7) for FSH [median 9.7 mlU/ml (range 4.6-37.8) vs. 21.7 mlU/ml (range 12.2-56.9)] and on D2 for LH [median 13.6 mlU/ml (range 6.8-25.2) vs. 21.2 mlU/ml (range 13-54.7)]. E<sub>2</sub> concentrations rose and reached their peak values on D3 [median 723 pmol/l (range 517-1,247.8) vs. 110.1 pmol/l (range 68.6-227.5) on DO]. These changes were followed by a subsequent rise in LH on D4 [36.7 mlU/ml (range 19.4-77.7)]. LH response to LHRH was higher during E<sub>2</sub> treatment: median value of absolute peaks: 156.3 mlU/ml (range 56.7-188.6) on D4 vs. 64 mlU/ml (range 38.9-131) on DO. These results demonstrate the presence of a positive feedback in patients with Klinefelter’s syndrome.