A total of 338 infants of gestational age 34 weeks or less were scanned regularly with real time ultrasound. Definite intraventricular haemorrhage was present in 126 (37%) infants, of whom 17 (13.5%) showed extension in the size of the initial haemorrhage, mostly into the cerebral parenchyma. These 17 infants were carefully matched with 17 others who had an initial haemorrhage of the same size and the same number of adverse perinatal factors. On analysis those infants with extension of intraventricular haemorrhage were constantly more acidotic and more anaemic than the control group. It is possible that careful attention to maintaining optimal condition after the onset of intraventricular haemorrhage may reduce the risk of extension.