OBJECTIVE: Our purpose was to determine whether second-trimester dimeric inhibin A levels distinguish Down syndrome pregnancies from euploid pregnancies. STUDY DESIGN: With use of a commercially available enzyme- linked immunosorbent assay (Serotec, Oxford) inhibin A medians were established in stored sera from 40 to 50 euploid pregnancies at each week of gestation from 14 to 20 weeks and from 33 Down syndrome pregnancies. Maternal serum α-fetoprotein, unconjugated estriol, and human chorionic gonadotropin levels measured in each sample before storage were retrieved. The performance of inhibin A in the multiple-marker screening test was evaluated. RESULTS: The mean inhibin A multiple of the median was significantly higher in the Down syndrome group than in the euploid group (2.84 ± 2.0 vs 1.22 ± 1.0, p = 0.0001). An inhibin A level ≤1.6 multiples of the median identified 70% of all Down syndrome pregnancies at a false-positive rate of 22%. Replacing estriol with inhibin A in the multiple-marker screening test resulted in a higher Down syndrome detection rate at a lower screen-positive rate. CONCLUSION: Elevated second-trimester maternal serum inhibin A levels identify Down syndrome pregnancies; replacing estriol with inhibin A in the multiple-marker screening test improves test performance.