In a survey of pregnancies, in which a chromosomal abnormality was diagnosed by amniocentesis but in which the mother did not undergo an abortion, the spontaneous fetal death rate after amniocentesis for 21 fetuses with Down's syndrome was 24 per cent (or 21 per cent, considering 19 singletons only). This rate is significantly greater than the rate of 2.7 per cent among 73 fetuses with less seriously abnormal genotypes reported in this survey or of 3.3 per cent and 3.5 per cent in the Canadian and United States studies of amniocentesis. After midtrimester there is about a sixfold higher risk of spontaneous fetal death for a fetus with Down's syndrome than for one with a normal genotype. At least some (and probably a major fraction) of the discrepancy observed between maternal-age-specific rates of Down's syndrome in live births and those found after amniocentesis is due to spontaneous fetal loss. A similar inference may be drawn for trisomy 18.