In a previous study of preterm infants requiring mechanical ventilation for the respiratory distress syndrome, we demonstrated a striking association of fluctuating cerebral blood-flow velocity in the first day of life with the subsequent occurrence of intraventricular hemorrhage. Because this fluctuating pattern could be eliminated by muscle paralysis, we conducted a prospective study of preterm infants receiving mechanical ventilation for the respiratory distress syndrome in which we evaluated the effect of paralysis and this flow-velocity pattern on the incidence and severity of intraventricular hemorrhage. Twenty-four infants with the fluctuating pattern in the first hours of life were identified and randomly selected to serve as controls (10) or to be subjected to muscle paralysis (14). Intraventricular hemorrhage developed in all 10 control infants but in only 5 of the 14 infants subjected to muscle paralysis. Moreover, in 4 of the 5 paralyzed infants in whom hemorrhage developed, it did so after cessation of the paralysis. Seven of the 10 control infants had Grade III hemorrhage, the most severe variety of intraventricular hemorrhage, whereas none of the paralyzed infants had Grade III hemorrhage. We conclude that elimination of fluctuating cerebral blood-flow velocity in preterm infants with respiratory distress syndrome markedly reduces the incidence and severity of intraventricular hemorrhage.