Proportions of infants of birthweight 501–2000 g born in 1979–1981 who had defects recognisable at birth were ascertained by examining hospital records and tracing survivors. Twenty-seven per cent of 230 stillbirths, 8.4% of 1411 livebirths, 4.5% of 1117 neonatal survivors and 3.6% of 1074 two-year survivors had a congenital defect, birth prevalence rising with lower birthweight only in stillbirths. Among livebirths, birth prevalence was increased in growth retarded infants, in singletons and in infants of mothers who had not had a previous reproductive loss. Twenty-four per cent and 57% of neonatal deaths in the 1001–1500 g and 1501–2000 g birthweight groups respectively were associated with a congenital defect. Major neurological impairments were directly attributable to congenital defects in 10% of 71 survivors; surgery and hospital admissions were far more frequent in infants with congenital defects than others. The 1501–2000 g group accounted for 77% of all survivors with a congenital defect. Further improvements in the prognosis of low birthweight infants will depend to a large extent on prenatal prevention of disease.