A new method of quantifying cerebral blood flow which allows repeated cotside measurements is described. 31 observations were made on nine ill, mostly very preterm, infants. Cerebral blood flow was usually about 18 ml.100 g-1.min-1, but ranged from 7 ml.100g-1.min-1 (after the administration of indomethacin to a very preterm infant) to 33 ml.100 g-1.min-1 in a birth-asphyxiated post-term infant.