Abnormalities detected by a mechanical sector scanner were compared 'blind' with autopsy findings in the brains of 56 infants born at less than 33 weeks gestation. Intraventricular haemorrhage was found in 53 of 112 hemispheres and had been accurately diagnosed by ultrasound (sensitivity 91 per cent; specificity 81 per cent). Isolated germinal layer haemorrhage was less successfully identified (sensitivity 61 per cent; specificity 78 per cent); false-positive diagnoses were partly due to difficulty in distinguishing haemorrhage from the normal choroid plexus in extremely preterm infants. Haemorrhagic parenchymal lesions were correctly identified in nine infants (sensitivity 82 per cent; specificity 97 per cent). Only 11 of 39 hemispheres with histological evidence of hypoxic-ischaemic injury, without marked bleeding, were correctly identified by ultrasound (sensitivity 28 per cent), mainly because of failure to detect small areas of periventricular leucomalacia and diffuse gliosis. 10 hemispheres with periventricular echodensities thought to represent leucomalacia showed no histological evidence of hypoxic-ischaemic injury (specificity 86 per cent). Ventricular dilatation in seven infants was always associated with evidence of hypoxic-ischaemic injury at autopsy.