Certain CT and/or MRI abnormalities have been used medicolegally to time intracranial injuries from the infant shaken impact syndrome (ISIS). For example, parenchymal hypodensities on CT scans are said to arise only after 6-48 h have elapsed postinjury, and the presence of chronic or mixed subdural hematomas suggests injury that occured 1-4 weeks prior. However, these statements are based largely upon inference from data obtained in other conditions such as ischemic anoxic injury and chronic subdural hemorrhage in adults. Direct evidence about the evolution of intracranial injuries in infants with ISIS is sparse, and the radiographic changes following ISIS have never been systematically studied on serial imaging studies. One hundred-seventeen serial CT and MRI scans obtained from 33 infants with ISIS were reviewed retrospectively. The exact scan dates and times were obtained directly from the scans. Acute subdural hemorrhage was the most common intracranial abnormality and was present in 27 (81%) of the 33 infants. Other intracranial abnormalities included chronic subdural collections, subarachnoid hemorrhage, epidural hematomas, parenchymal hypodensities, edema and contusions, and atrophy and encephalomalacia. In 15 of the 33 infants, the injury could be timed with reasonable certainty, and the evolution of the radiographic changes followed over time. Six of the 15 infants had evidence of prior cranial trauma such as chronic subdural collections (5 infants) or mild atrophy (1 infant). Of the remaining 9 infants, parenchymal abnormalities such as hypodensities, edema and contusion appeared in virtually all of the initial scans performed approximately 3 h following the report of injury. One 'chronic' subdural collection was absent on the first scan performed 2.75 h postinjury, but appeared on a second scan performed 17 h later, suggesting that some 'chronic' subdural fluid collections may arise much sooner than previously thought. These findings challenge some of the current dogma about the timing of radiographic changes following abuse and are important in timing the alleged abuse for legal purposes.