Cytogenetic studies were performed in 500 couples referred for in-vitro fertilization or gamete (zygote) intra-Fallopian transfer. Thirteen individuals (1.3%) with chromosomal abnormalities were found. Four major types of anomalies were observed: reciprocal translocations (n = 3), inversions (n = 2), iso-Xq chromosomes (n = 2) and sex chromosome number mosaics (n = 4). Moreover two males with respectively a 47,XYY and a 47,XY,mar+ karyotype were identified. These data pointed to a higher incidence of chromosomal aberrations in this infertile population as compared to a neonatal population without obvious chromosomal pathology. Analysis of the chromosomes which were involved in hyperdiploidy and hypodiploidy in the 30,000 metaphases evaluated, showed a high proportion of cells that had lost or gained an X-chromosome. A puzzling finding was the statistically significant low incidence of 45,X metaphases (0.9%) in women of couples treated on andrological indication as compared to the frequency of 45,X chromosome complements in women with tubal disease (4.0%) or of couples with an idiopathic (4.3%) or mixed female and male (6.7%) indication.