Objectives: To determine the incidence of chromosome abnormalities among couples for whom intracytoplasmic sperm injection (ICSI) treatment was indicated and fetuses conceived through the ICSI procedure. Methods: All cytogenetic results were evaluated retrospectively. Patients undergoing ICSI (n = 508) were classified according to the referring indications as: (1) males with severe infertility ( 87 azoospermia and 34 oligoasthenoteratozoospermia, OAT), (2) prior to ICSI (56 males and 61 females), and (3) following an unsuccessful ICSI procedure ( 132 males and 138 females). Fetuses conceived through ICSI (n = 475) were also classified into 4 groups according to the additional risk factors for chromosome abnormalities: ICSI (n = 185), ICSI + advanced maternal age (AMA, n = 215), ICSI + positive triple test result (TT, n = 50), and ICSI + abnormal ultrasound findings (USG, n = 25). Results: An abnormal karyotype was found in 31.03% of males with azoospermia and 14.71% of males with OAT, in 3.57% of males and 1.64% of females in the group prior to ICSI, and in 5.30 and 5.07%, respectively, in the group following unsuccessful ICSI treatment. Gonosomal aneuploidies were predominant in males with azoospermia and autosomal rearrangements in males with OAT, while low-level sex chromosome mosaicism was found in females. The overall frequency of chromosome abnormalities in fetuses was 4.42% and varied in the different groups from 1.62% in ICSI, 2.79% in ICSI + AMA, 10.0% in ICSI + TT to 28.0% in ICSI + USG. The frequencies of the different types of chromosome abnormalities were as follows: balanced 1.05%, unbalanced 3.37%, familial 0.84%, de novo 3.37%, autosomal 3.58%, gonosomal 0.84%, numerical 1.89%, structural abnormalities 2.53%, and mosaicism 1.26%. Conclusion: Our results indicate that cytogenetic investigations of the ICSI parents and fetuses are essential for the families, genetic counselors and also reproductive centers. In fetal karyotyping, de novo structural chromosome abnormalities and mosaicism should be taken into consideration. Copyright (C) 2004 S. Karger AG, Basel.