The Cerebral Palsy Register in eastern Denmark has collected cases using a uniform data sampling procedure since birth year 1979. We have investigated changes in the rate of cerebral palsy, related to gestational age, mortality and perinatal risk factors in children born 1983--90. The total cerebral palsy birth prevalence decreased from 3.0 in the birth year period 1983--86 to 2.4 per 1000 live births (P < 0.01) in 1987--90, owing to a decrease among all preterm infants (29--19 per 1000, P < 0.001). The perinatal and early neonatal mortality in preterm infants was unchanged from 1983--86 to 1987--90. The rate of cerebral palsy in term infants was 1.5 per 1000 in all birth-year periods from 1979--90. Among the cerebral palsy infants, the proportion of very preterm babies treated with mechanical ventilation in the neonatal period decreased from 95% in 1983--86 to 61% in 1987--90 (P < 0.001), while the group treated with CPAP among the moderately preterm babies increased from 61% to 78% (P < 0.05). The significant decline in cerebral palsy rate in preterm infants born 1987--90 may be due to a change in treatment at the neonatal intensive care units using less mechanical ventilation, a hypothesis which needs further investigation.