Chorion villi sampling (CVS) in the first trimester of pregnancy has become available recently as an alternative method to second trimester amniocentesis for prenatal diagnosis of genetic defects. Currently there are six different tissue sampling techniques being investigated in different centers around the world, but there are very few trials in ongoing pregnancies. From chorionic villi material cytogenetic and biochemical studies as well as DNA analyses can be performed. Different methods of chromosome analysis are being investigated at the University of California San Francisco and elsewhere to determine the most efficient and reliable techniques. Larger studies will be needed to establish the efficacy and the safety of the chorionic villi sampling procedure for the mother and the developing fetus. Although CVS is preferable to amniocentesis for psychological and medical reasons (earlier elective termination of a fetus with a genetic disorder), many questions remain to be answered in carefully controlled studies.