With the development and routine use of real-time ultrasound scanning, it has been possible to collect 20 autopsy cases of infants in whom the ultrasound scan was first normal and then showed evidence of subependymal/intraventricular hemorrhage (SEH/IVH). Analysis of these cases with known postnatal origin of the hemorrhage permitted the development for the first time of a time scale to characterize the temporal evolution of the histopathologic reactions to hemorrhage in the premature human brain. This time scale was then used to define the histologic stage of the lesions in 27 other cases of infants who had a scan demonstrating SEH/IVH prior to death but who had not a previously normal scan. Only 2 cases were found to lie off the original scale, indicating a prenatal onset of the hemorrhage in about 5% of the total cases. Thus, postnatal events immediately preceding the onset of the hemorrhage are more likely to be implicated in the pathogenesis of SEH/IVH than prenatal or obstetric events. Comparison with similar reactions in the adult indicate that the early reactions by macrophages occur at about the same rate but the routine transfer of iron from macrophages to astrocytes and the much slower rate of absorption to form a cyst in the adult combine to form a complicated pattern that requires a different set of criteria for the intermediate and late stages in the adult.