The authors evaluated the risk of chorion biopsy used within the framework of prenatal diagnosis of the foetus. The incidence of abortions following transabdominal CVS (0.6%) did not differ from abortions after amniocentesis in the second trimester (0.5%). In a group of 1002 diagnostic biopsies of the chorion the authors did not record in the born infants reduction deformities of the extremities. Chorion biopsies were performed in 98% between the 10th and 12th week of gestation. The incidence of mosaicism--1.59%--was not significantly higher than reported in the literature. The authors recommend: to increase the size of the sample of chorion tissue by the use of a manual aspirator, the use of a 30 ml plastic syringe or the double needle method. They consider CVS a suitable method which serves the diagnosis of IUGR and the prenatal diagnosis of the foetus before the 20th week of gestation when cordocentesis involves risk.