In three centers in Copenhagen prenatal chromosome analysis has been carried out on 93 fetuses. Forty patients had earlier born a child with Down's syndrome. One had a 47, XY,+D trisomy fetus. Twenty were 40 years of age or more. Two fetuses in this group had 47, XY,+21 trisomy and one patient had a spontaneous abortion, which was found to have the same constitution. in four of five pregnancies with familial translocations the fetus was a translocation heterozygote. in five cases with relatives affected with Down's syndrome, in seven where the mother had earlier born a child with a chromosome anomaly not being +21 trisomy and in seven at risk of various inborn errors of metabolism, the fetuses were normal with regard to the chromosome constitution. Seven of nine cases, which were at risk of having an X-linked disease, were males. Five of these were therepeutic aborted, one denied abortion and had an affected male child. One male fetus had a haemophilic father and the family did not want another carrier female.