This study reports the associations between antenatal complications, subnormal urinary oestriol excretion and perinatal death in 500 pregnancies when the baby weighed less than the 10th centile for gestational age at birth, compared with those in a series of 500 pregnancies when the baby was of a normal weight for gestation. The overall incidence of antenatal complications was not higher in those pregnancies in which the fetus was growth retarded, although early onset pre-eclampsia, threatened abortion, diabetes mellitus and accidental haemorrhage were commoner (P less than 0.05). The incidence of subnormal urinary oestriol excretion was significantly higher in pregnancies in which the fetus was growth retarded, both when other antenatal complications were present (54.7% in the study group, 18.4% in the control group P less than 0.001) and in uncomplicated pregnancies (37.7% and 13.3%, respectively, P less than 0.001). Subnormal oestriol excretion identified 20 of the 26 perinatal deaths in the growth retardation group and 4 of the 6 perinatal deaths in the control group. Perinatal mortality was 10 times higher in growth retarded infants than in infants of appropriate size for gestation when pregnancy was not complicated antenatally. This study confirms the need to identify the presence of fetal growth retardation antenatally to enable appropriate treatment and improvement in perinatal mortality. The presence of antenatal complications is not appropriate for identification, whilst subnormal urinary oestriol excretion was seen to have highly significant predictive value.