Objective: To report the initiation of a pregnancy that was achieved by intracytoplasmic sperm injection (ICSI) with sperm from a patient with Klinefelter's syndrome. Design: Case report. Setting: University women's hospital IVF center. Patient(s): A couple with primary infertility and nonmosaic 47,XXY karyotype of the male partner. Intervention(s): Intracytoplasmic sperm injection after ovarian stimulation and transvaginal ultrasound-guided oocyte pick-up with sperm from a hypergonadotropic man with a nonmosaic 47,XXY karyotype. Main Outcome Measure(s): Clinical pregnancy. Result(s): Despite a 47,XXY karyotype in all 50 analyzed lymphocyte metaphases, the sperm of the patient led to a clinical pregnancy with the first attempt of ICSI and intrauterine transfer of three embryos. The pregnancy stopped developing in the ninth week. Cytogenetic investigation of the abortion material revealed a numerical normal 46,XX karyotype. Conclusion(s): Sperm from a patient with hypergonadotropic nonmosaic Klinefelter's syndrome, when used for ICSI, can lead to a pregnancy. (C) 1997 by American Society for Reproductive Medicine.