The aim of this study was to analyse the frequency of sex-chromosomal aneuploidy in human spermatozoa of severe oligozoospermic men undergoing intracytoplasmic sperm injection (ICSI), to evaluate the impact of these chromosomal anomalies on the results of the ICSI. Fluorescence in situ hybridization (FISH) with direct label fluorescence DNA probes specific for chromosome X, Y and 18 was performed on decondensed spermatozoa from fresh ejaculates of 12 patients with severe oligozoospermia undergoing ICSI. A total of 500 spermatozoa were analysed per donor. The rate of gonosomal aneuploidy was significantly increased in the patients compared with normal donors (3.5% and 0.8% respectively). The sex-chromosomal anomalies due to meiosis I (XY) are less important than the anomalies caused by meiosis II (XX or XY), but the difference was not statistically significant. There was a negative correlation between the rate of aneuploidy and the percentage of spermatozoa with normal morphology (r = -0.71; P < 0.05). The correlation was negative between the percentage of gonosomic aneuploidy and the rate of fertilization (r = -0.7; P < 0.001). Our results suggest an increased rate of gonosomic aneuploidy in the patients with oligozoospermia compared with the normal population. This aneuploidy, although it decreases the rate of fertilization, does not seem to affect the rate of cleavage, nor the embryonic quality.