Fourteen hydrocephalic children were studied who were between the ages of 6 months and 14 years. Regional cerebral blood flow (rCBF) was measured by the 133Xe intravenous injection method after ventriculoperitoneal shunting. There was a negative correlation between slow flow and preoperative ventricular size (r = -0.718, P < 0.02), but there was no correlation between fast flow and preoperative ventricular size. There was also no correlation between rCBF and postoperative ventricular size. Postoperative IQ or development quotient showed a positive correlation with slow flow (r = 0.813, P < 0.01), but not with fast flow. It is suggested that in hydrocephalic children there is impairment of white-matter communicating fibres and secondary reduction in higher intellectual activity.