This study reports the fetal outcome in 500 pregnancies when the baby weighed less than the 10th centile for gestational age at birth, compared with that in a series of 500 pregnancies where there was a normal weight for gestation. Fetal growth retardation (0-9th centile) had a significant positive association with perinatal mortality (5.2% versus 1.2%, P greater than 0.001) and low oestriol excretion (42.4% versus 15%, P greater than 0.001), but not with major fetal malformations or fetal asphyxia. In the study group, 20 of the 26 perinatal deaths were associated with subnormal oestriol excretion. When severe fetal growth retardation was considered (less than the 5th centile), the associations with perinatal mortality (19%) and subnormal oestriol excretion (63%) were stronger and a significant correlation with major malformations emerged (17%, P greater than 0.001). Detection of subnormal oestriol excretion allows identification and appropriate treatment of severe fetal growth retardation which should improve survival and neurological development in these infants. This study confirms that birth-weight below the 10th centile is an appropriate definition of fetal growth retardation in terms of perinatal mortality and morbidity.