In the context of the aetiological investigation of male infertility, the authors stress the place and the contribution of blood karyotype testing in the light of their personal experience based on 1,612 subjects. This examination has an important place, as about 15% of azoospermic subjects and 6 to 7% of subjects with oligospermia less than 10 million spermatozoa per ml, either alone or in combination with other abnormalities of the semen examination, present a congenital chromosomal abnormality. A remarkable constancy of the results was observed according to identical recruitment criteria. The contribution of this examination is also important: the medical and psychological value of detecting the cause of azoospermia, genetic counselling and antenatal chromosomal diagnosis for non-azoospermic subjects with an equilibrated structural abnormality, in whom treatment allows a chance of procreation, genetic counselling for the family of these subjects in order to prevent the appearance of a chromosomally abnormal infant. In conclusion, the authors argue in favour of the routine use of this test in all infertile subjects with at least isolated oligospermia less than 10 million spermatozoa per ml.