Serial weekly measurements of somatic growth and head circumference were made for ten weeks on 41 appropriately grown sick preterm infants (gestational age 28 to 32 weeks) with severe neonatal problems. Twenty-seven (66%) required prolonged assisted ventilation. During the period of acute illness, the velocity of growth for the sick infants was below that of the normal fetus, with deviation away from and below the normal fetal growth curve. During recovery, head growth paralleled that of normal fetal growth, and subsequently rapid "catch-up" growth in head circumference occurred. By comparison, six similar infants, whose head circumference followed the intrauterine growth curve, proved to have hydrocephalus. These results suggest that: (1) the brain participates in the growth retardation associated with being sick and premature and that apparently normal head growth under comparable circumstances may be associated with hydrocephalus; (2) in spite of an energy intake ranging from 80 to 120 kcal/kg/day by the end of the second postpartum week, normal growth in the sick low-birth-weight infant does not occur until their acute illness has resolved.