This case:control study demonstrates that although the incidence of spastic cerebral palsy is falling in Western Australia, the contribution made by low-birthweight and small-for-dates infants who have significant neonatal morbidity, and who have been treated in neonatal intensive care units, has increased significantly. This points to factors other than care being important in diminishing the incidence of spastic cerebral palsy. Further falls in incidence may only result from improvements in the management of the very small and very sick neonate, and preventive programmes aimed at preterm births. Obstetric intervention and fetal distress were more frequent in recent cerebral-palsy cases than in the controls. Thus, falling neonatal mortality among low-birthweight infants in Western Australia coincided with an increased incidence of spastic cerebral palsy in these infants, despite an over-all drop in the incidence of cerebral palsy.