The incidence, timing, severity, and outcome of intraventricular hemorrhage (IVH) were studied in extremely small premature infants with birth weights (BWs) between 500 and 700 g; 366 infants with BWs between 701 and 1500 g, admitted during the same period, served as a comparison group. Intraventricular hemorrhage occurred in 34 (62%) of 55 infants with BWs less than 700 g vs 91 (25%) of the 366 comparison infants. In the group with BWs less than 700 g, IVH occurred in the first 18 hours, from 19 to 72 hours, and after 72 hours of life in 62%, 20%, and 18% of the infants, respectively. In the comparison group, the occurrence for these periods was 13%, 82%, and 5%, respectively. The severity of IVH in infants with BWs less than 700 g was grade III (with or without intraparenchymal hemorrhage) in 97% of the lesions, but in the comparison group such severe IVH accounted for only 32% of the lesions. Intraventricular hemorrhage was a common contributor to death in the infants with BWs less than 700 g. Thus, in 24 infants who died before 72 hours of life, 21 infants (88%) had severe IVH. In addition, intracranial hemorrhage (four infants with IVH and two infants with intracerebellar hemorrhage) occurred late (days 8 to 25) and contributed to death in six of the infants with BWs less than 700 g. These data indicate that in comparison with larger premature infants, infants with BWs less than 700 g exhibit a higher incidence of IVH, which is more severe, occurs earlier, and is associated more often with a fatal outcome. In addition, late and lethal intracranial hemorrhage is also more likely to occur in these smaller infants.