The use of chromosome specific DNA probes labelled with fluorochromes and especially the combination of several probes has been used to indirectly study the chromosome constitution of decondensed sperm nuclei by fluorescence in-situ hybridization (FISH), and has allowed to include this test in the protocol of study of infertile males. Still, if the test is to be valid, several strict conditions must be met, and some specific characteristics have to be taken into account. This becomes evident when comparing earlier results with more recent ones. The basic technical factors to be taken into account are the methods of chromatin decondensation, the number of spermatozoa and of individuals to study, the use of internal controls, the scoring criteria, the specificity of the probes and the possible existence of polymorphisms that may interfere with the detection of fluorescent signals. In the last 7 or 8 years, a large number of papers has been published, describing the incidence of aneuploidies in controls, in individuals in whom a tendency to non-disjunction was suspected and in infertile males. Studies in controls have shown a considerable intra- and inter-individual variability in the frequency of aneuploidies, the tendency of some chromosomes to undergo non-disjunction (chromosome 21 and the sex chromosomes) and the importance of alpha-satellite polymorphisms when using centromere probes. In the control population, the frequency of aneuploidy per haploid set has been estimated at approximately 6%. The incidence of aneuploidies in sperm nuclei for some of the chromosomes more frequently involved in trisomies is considerably higher than the incidence of these trisomies established through epidemiological data using the global incidence of chromosome abnormalities during the peri-implantation stage. In infertile males and in males with sex-chromosome abnormalities (usually with very low numbers of spermatozoa) the results show an increased incidence of sex chromosome aneuploidies and diploid (multi-aneuploid?) sperm nuclei. The results could be related to the higher incidence of chromosome abnormalities (especially sex-chromosome aneuploidies) observed in children conceived by intracytoplasmic sperm injection (ICSI).