Chorionic villi sampling at week 9–11 of gestation is increasingly applied as a new method in prenatal diagnosis of genetic disorders. Cytogenetical, biochemical or recombinant DNA methods have been used successfully in more than 6000 pregnancies at risk. Different obstetric techniques for sampling chorionic villi using a catheter or a biopsy forceps, both under ultrasound guidance only, are practised so far. At the present stage of experience main problems of this method seem to be the risk of foetal loss, diagnostic errors caused by maternal cell contamination, as well as the difficulty in the interpretation of chromosomal mosaicism.