Among 19 infants in whom cerebral blood flow had been determined a few hours after birth, four died during the first days or weeks after birth, all with massive intracranial hemorrhage. The other infants were examined at 9 to 12 1/2 months of age by means of clinical neurologic evaluation, developmental psychologic assessment (Cattell), EEG, and cranial computed tomography. Six of the ten infants who had had CBF of 20 ml/100 gm/minute or less had developed cerebral atrophy as demonstrated at autopsy or by CT scan, none with neonatal flows above 20 had done so. Only one in the low flow group had developed completely normally, whereas abnormal development was found in only a minority of the high flow group. No other neonatal observation had such a clear relationship to later development. It is concluded that CBF of 20 or less during the first hours of life is critical.