To establish the effect of antenatal vitamin K on the incidence and severity of intraventricular hemorrhage, 92 patients destined to deliver infants less than 32 weeks' gestation were, in a prospective fashion, randomly assigned to two groups. Group I received 10 mg of vitamin K1 intramuscularly every 5 days until delivery. Group II received no antenatal vitamin K1 therapy. There were 100 neonates less than 1500 gm, equally divided between groups I and II with respect to gestational age, birth weight, race, sex, presentation, route of delivery, duration of labor, Apgar scores, and arterial umbilical cord acid-base balance. The antenatal use of vitamin K resulted in significant reduction in the prothrombin time (12.7 versus 15.2 seconds) and partial thromboplastin time (42.6 versus 58.9 seconds; p less than 0.05). Furthermore, group I experienced a lower incidence of total (16% versus 36%) and severe (0% versus 11%) grades of intraventricular hemorrhage (p less than 0.05). This study suggests that the antenatal use of vitamin K may result in a reduction and severity of intraventricular hemorrhage in the neonate less than 1500 gm.