Data on detection of chromosomal mosaicism in amnionic cells and chorionic villi obtained by prenatal cytogenic diagnosis are presented. The frequency of chromosomal mosaicism in preparations of amniotic fluid cell culture was 2.6% (6 out of 226), and that in "direct" villus preparations was 1.6% (13 out of 774). The necessity perform an additional analysis of other fetal cells or neonatal lymphocytes to specify the diagnosis was shown. The analysis of the outcome of pregnancies during which chromosomal mosaicism in the extraembryonic cells was detected indicates that these women from a high-risk group of at, both genetically and obstetrically: in only 8 out of 19 cases did pregnancies end in normal deliveries at term; in three cases, spontaneous abortions occurred at 16-31 weeks of gestation; in three cases, the pregnancies were terminated due to fetal chromosomal aberrations in nonmosaic form; the outcome of pregnancy in five cases was preterm delivery of an underweight newborn.