With the advent of modern assisted reproduction techniques, especially with the intracytoplasmic sperm injection (ICSI) effective treatment has become available for men with severe male infertility. ICSI is efficiently used in clinical practice, but unfortunately, as a result of intensive research an increased risk of transmission of cytogenetic defects to the offspring has also been documented. With the cytogenetic studies of semen from 32 infertile men, we documented, that in oligospermic patients who are candidates for ICSI, there is an increased frequency of sperm with sex chromosome aneuploidy, especially the XY disomy. Further, the diploidy frequency is increased in oligozooastenospermic samples. Defective chromosome separation of either meiosis I. or meiosis II. can be responsible for the production of diploid spermatozoa, but in patients with low sperm count the failure of the nuclear cleavage during meiosis II. seems to be responsible to the elevated diploidy frequency. Conventional WHO parameters of semen analysis (sperm count and motility) do not correlate with the frequency of numerical chromosomal anomalies. The risk can be determined using fluorescence in-situ hybridization (FISH) on decondensed spermatozoa. Due to the elevated risk of transmission of genetic disorders to the offspring with ICSI, there is a need to eliminate spermatozoa with numerical chromosomal anomalies before assisted fertilization. With the FISH examination of over 470,000 sperm from 44 donors we established, that presently used sperm preparation techniques (the gradient centrifugation and swim-up) are not sufficiently effective in eliminating both aneuploid and diploid sperm. The gradient centrifugation effectively decreases the frequency of immature and aneuploid sperm, while the swim-up preparation is able to reduce significantly the diploid sperm due to their defective motility. There is a correlation between the rate of immature sperm and aneuploidy frequency, but no relationship seems to exist among sperm motility and aneuploidy. Also, selecting sperm for ICSI, based on shape properties alone, does not preclude the presence of chromosomal abnormalities, particularly disomies. Using objective morphometry and FISH on the same sperm we gave evidence, that sperm with chromosomal aberrations may occur among normal spermatozoa. We developed a sperm selection method based on the membrane properties and hyaluronic acid binding capacity of mature spermatozoa. Only mature sperm with low aneuploidy/diploidy frequencies are able to bind the solid state hyalunonan. Sperm selection with our experimental method may provide a new, safe and efficient solution for selection of individual mature sperm for ICSI with very low risk of numerical chromosome abnormalities.