The study was performed to determine factors affecting successful sperm retrieval by testicular sperm extraction in patients with nonmosaic Klinefelter's syndrome (KS). From May 2001 to February 2007, 27 azoospermic patients were diagnosed as having nonmosaic KS. All patients underwent sperm testicular extraction. Patient's age, testicular volume, serum follicle-stimulating hormone (FSH) and inhibin B were assessed as predictive factors for successful sperm recovery. Of the 27 Klinefelter's patients examined, eight (29.6%) had successful sperm recovery. The comparisons of serum FSH, inhibin B and testicular volume between patients with and without successful sperm retrieval did not show any statistical significance. The patients with successful sperm recovery were significantly younger (28.6 +/- 3.11 years) than those with failed attempts (33.9 +/- 4.5 years, P = 0.002). The rate of positive sperm retrieval was significantly higher in patients younger than 32 years compared with patients older than 32 years (P = 0.01, chi-squared test). The study showed that clinical parameters such as FSH, inhibin B and testicular volume do not have predictive value for sperm recovery in patients with KS. The mean age of our patients with successful sperm recovery was significantly lower than that of men with unsuccessful results. Testicular sperm extraction or testicular sperm aspiration should be performed before the critical age of 32 years.