Fetal growth restriction (FGR) is a clinically significant pregnancy disorder in which the fetus fails to achieve its full growth potential in utero. Most cases of FGR are idiopathic and are associated with placental thrombosis. Previous studies suggest that proteoglycans, such as decorin, that contain the glycosaminoglycan dermatan sulfate are the principal anticoagulants in the normal placenta. The present study investigated decorin expression in placentas from pregnancies complicated by idiopathic FGR (n = 26) and gestation-matched controls (n = 27). Real-time polymerase chain reaction demonstrated significantly reduced decorin mRNA expression in FGR compared with control (1.52 ± 0.14 v. 2.21 ± 0.22, respectively; P < 0.01). Immunoblotting revealed decreased decorin protein (40 kDa) expression in FGR compared with controls (420.8 ± 39.0 v. 690.1 ± 42.2, respectively; n = 12 in each group; P = 0.0007). Immunohistochemistry demonstrated the presence of immunoreactive decorin protein in the placental villous stroma surrounding the fetal capillaries and a significant decrease in decorin protein presence in FGR compared with control (1.75 ± 0.66 v. 2.98 ± 1.12, respectively; n = 6 in each group; P < 0.01, t-test). This is the first study to demonstrate reduced decorin in idiopathic FGR, indicating a potentially significant role for decorin in the aetiology of placental thrombosis in idiopathic FGR.