A prospective study of 377 premature infants (less than or equal to 1500 gm) was undertaken to delineate the natural history of subependymal/intraventricular hemorrhage (S/IVH) and its complications using ultrasound (US) and computed tomography (CT). Low grade (I, II) S/IVH had a low mortality while higher grades (III, IV) still had elevated mortality rates. The addition of intraparenchymal hemorrhage (IPH) to S/IVH incrementally increased the incidence of death and other complications, suggesting IPH hemorrhage should be categorized separately. When a specific day could be identified, S/IVH had its onset in the first 7 days of life with peak incidence occurring on day 3. S/IVH appeared to be an event limited to less than 24 hours in all but 5% of infants in whom progression of hemorrhage was documented over a 24-hour period. The mortality rate of these progressive hemorrhages was high, 50%. The benign phenomenon of late S/IVH was detected in 5% of infants. These hemorrhages were clinically silent and of minor severity. Several complications of S/IVH were detected. Hydrocephalus was a significant complication only for higher grades of S/IVH. When present, severe hydrocephalus had an early onset and reached a maximum at around 3 weeks of age. "Atrophic change" of a cerebral hemisphere was detected in 30% of all S/IVH infants, while this was not seen in nonS/IVH infants. This "atrophic" abnormality had a marked predilection for the left hemisphere, independent of the site of the S/IVH. Periventricular leukomalacia (PVL) was documented by US in 2% of infants and could be detected in the first week of life. PVL presented in the first week of life as an echogenic lesion which developed "cystic" changes at approximately 3-4 weeks of age. This complication should be categorized separately from S/IVH.