To investigate changes in cerebral palsy birth prevalence and perinatal mortality rate by different gestational age groups, 1979-86, cerebral palsy cases in eastern Denmark were identified from the Danish Cerebral Palsy Register, and information on birth and mortality rates was sought in the Danish Medical Birth Register. From 1979-82 to 1983-86, the birth prevalence of cerebral palsy increased from 2.6 to 3.0 per 1000 (P < 0.05). The rate for infants of 31 weeks' gestation or more did not change, whereas a significant increase was observed in infants below 31 weeks (85-123 per 1000, P < 0.05). In the same periods, perinatal mortality in eastern Denmark decreased significantly from 8.6 to 7.8 per 1000. The decrease in stillbirth rate was significant in all subgroups of gestational ages except in those of 28-30 weeks' gestation. The early neonatal mortality rate decreased significantly only in infants below 31 weeks (282-239 per 1000, P < 0.05). Thus, in eastern Denmark, cerebral palsy birth prevalence has increased from birth-year period 1979-82 to 1983-86 because of an increased rate in preterm infants below 31 weeks, who at the same time had a reduced risk of early neonatal death.