The effects of neonatal illness and caloric intake on head growth velocity and on 1-year developmental outcome were studied in 73 appropriate (AGA) and small for gestational age (SGA) premature infants of (mean +/- SD) 30 +/- 2 weeks gestation who received intensive care during the neonatal period. Head growth from birth to 1 year of corrected age was characterized by a triphasic curve initiated by a period of growth arrest or suboptimal growth followed by a period of catch-up growth and terminated by a period of growth along standard curves. Head growth arrest or suboptimal head growth were directly related to the duration of the initial period of caloric deprivation (less than 85 kcal/kg/day) and to the duration of mechanical ventilation. Catch-up head growth was influenced by the duration of the preceding period of caloric deprivation in all infants and by the caloric intake during that phase only in SGA infants; catch-up growth was unrelated to the duration of mechanical ventilation. Head growth along standard curves occurred in all infants by 3 months of corrected age and persisted up to 1 year of corrected age. Infants calorically deprived the longest (AGA 4 to 6 weeks, SGA 2 to 3 weeks) had head growth along standards at curves below -1 SD on the growth chart; all other groups had this phase of head growth at curves between the mean and -1 SD. Infants calorically deprived for more than 4 weeks had developmental scores below normal ranges by 1 year of corrected age.