The proform of eosinophil major basic protein (proMBP), the most abundant protein in the eosinophil specific granule, is synthesized by the placenta and secreted into the maternal circulation, where it is found complex‐bound to pregnancy‐associated plasma protein‐A (PAPP‐A) and other proteins. We examined the potential of proMBP as a maternal serum marker for fetal Down syndrome (DS) by determining its maternal serum concentration (MSpMBP) in 25 Down syndrome (DS) pregnancies and 152 control pregnancies in the first trimester, and in 105 DS pregnancies and 156 control pregnancies in the second trimester. The median (95 per cent confidence interval) MSpMBP MoM in DS pregnancies (n=15) was 0.66 (0.49–0.79) in gestational weeks 5–9; 1.06 (0.71–1.97) in weeks 10–12 (n=10) and 1.62 (1.18–1.98) in weeks 14–20 (n=105). Using parameterized receiver operator characteristics analysis for proMBP as a single marker for DS, detection rates (DRs) of 22 per cent and 38 per cent, for false‐positive rates (FPRs) of 5 per cent, were found in weeks 5–9 (using MSpMBP⩽cut‐off) and weeks 14–20 (using MSpMBP⩾cut‐off), respectively. When age and MSpMBP were used as markers in combination, a DR of 36.8 per cent for an FPR of 5.5 per cent was obtained in weeks 5–9 using a risk cut‐off of 1:250. In weeks 14–20 the DR was 48.4 per cent for an FPR of 5.3 per cent using the same risk cut‐off. This makes proMBP a marker comparable in diagnostic efficiency to human chorionic gonadotrophin (hCG), and exceeding that of alpha‐fetoprotein (AFP) and unconjugated oestriol (uE3), in the second trimester. Copyright © 1999 John Wiley & Sons, Ltd.