Prenatal diagnosis in the first trimester was established in 318 pregnancies at risk by means of transcervical chorion villus sampling (TC-CVS). Criteria for exclusion were discrepancy between the crown-rump length (CRL) and the expected week of gestation, if there was simultaneously insufficient increase in CRL during a week. With the exception of two cases, (0.6%) the specimens of villi obtained were sufficient for diagnostic purposes (mean: 55 mg (8-200 mg]. Twenty-five patients underwent terminations of pregnancy on account of the abnormal findings and one for social reasons. The fetal loss following CVS was seven (2.4%). No serious maternal complications occurred. Vaginal bleeding presumably on account of perforation of the placenta by the catheter was seen in 8.5% (27) of the cases and does not appear to influence the continuing pregnancy. The cytogenic results were all based on the direct prepartion method and also on culture technique and these were successful in 100%. Two false positive results (0.6%) were found in the direct preparation but normal findings were obtained on culture. No false negative results were found. Late complications i.e. preterm delivery (4.6%), perinatal loss (0.7%) and neonatal death (0.7%) did not exceed the expected values. The mean birthweight was within the normal range. It is concluded that transcervical CVS with the Trophocan catheter seems to be a safe and easy way to obtain prenatal diagnosis and this could be offered on the same terms as amniocentesis.