The brains of 95 consecutively admitted infants born at less than 33 weeks gestation were scanned with ultrasound. Thirty-six (38%) had periventricular haemorrhage (PVH). Eight (8%) had cerebral atrophy (together with PVH in 5). Twelve (40%) of the 30 infants in whom satisfactory timing of PVH was possible bled on the first day of life, but the median age when PVH was first detected was during the second day. The median age when PVH reached its maximum extent was the fourth day. The most significant antecedents of PVH were very short gestation and the presence of severe respiratory illness, particularly hyaline membrane disease, necessitating mechanical ventilation. Analysis of variance showed that pneumothorax arising during ventilation was the single most significant antecedent of PVH. Other significant antecedents, notably prolonged coagulation times, were found almost exclusively in infants with severe respiratory illness. Cerebral atrophy appeared usually to be attributable either to severe asphyxia during delivery or later, or to the formation of cysts at the site of previous haemorrhage into brain tissue.