Low maternal serum α-fetoprotein levels have been associated with fetal aneuploidies. Amniotic fluid α-fetoprotein levels have been reported to be low with Down syndrome (trisomy 21) but not with other fetal trisomies. We compared the amniotic fluid α-fetoprotein levels from 25 cases of autosomal trisomy (18 of trisomy 21, four of trisomy 13, three of trisomy 18) diagnosed by midtrimester fetal cytogenetic studies with those from matched, cytogenetically normal pregnancies. With these normal pregnancies used as controls, statistical analyses were performed on the data for all the trisomic fetuses, on the data for trisomy 21 only, and on the data for trisomies 13 and 18 combined. Amniotic fluid α-fetoprotein levels were significantly lower in the 25 trisomic cases compared with controls, 0.77 ± 0.34 versus 1.03 ± 0.34 mg/dl (p<0.001). However, further analysis revealed that the difference was due to the trisomy 21 data alone. In the trisomy 21 cases there was a significant difference for α-fetoprotein levels between cases and controls (p < 0.001), whereas there was no difference for the combined trisomy 13 and 18 cases compared to controls (p > 0.40). These findings suggest that the low maternal serum levels of α-fetoprotein reported in cases of Down syndrome may be related to reduced amniotic fluid concentrations. However, the reduced maternal serum α-fetoprotein levels reportedly associated with trisomies 13 and 18 do not seem to be explained by low amniotic fluid concentrations. © 1985.