The umbilical venous oxygen and carbon dioxide tensions, pH, lactate and glucose concentrations, nucleated red cell (erythroblast) count, and haemoglobin concentration were measured in 38 cases of intrauterine growth retardation in which fetal blood sampling was performed by cordocentesis. The oxygen tension was below the normal mean for gestational age in 33 cases; in 14 it was below the lower limit of the 95% confidence interval for normal pregnancies. The severity of fetal hypoxia correlated significantly with fetal hypercapnia, acidosis, hyperlacticaemia, hypoglycaemia, and erythroblastosis. These findings indicate that "birth asphyxia" is not necessarily due to the process of birth.