Reports have supported an association between elevated midtrimester maternal serum alpha-fetoprotein concentrations (unexplained by fetal anomalies) and intrauterine growth retardation. Our observations show an association between such elevations of maternal serum alpha-fetoprotein levels and two types of placental pathology at delivery, chronic villitis and placental vascular lesions of infarction or intervillous thrombosis. If chronic villitis was present, the frequency of intrauterine growth retardation was significantly increased, whereas no increase in intrauterine growth retardation was found in the absence of placental pathology.