Bilateral convexity and interhemispheric subdural hematomas are common neuroimaging patterns seen in infants who have sustained nonaccidental head injuries (NAHIs). These collections often appear aschronic or acute-on-chronic on computerized tomography (CT) studies. To determine the nature of these extraaxial fluid collections and their relationship to cerebrospinal fluid (CSF) dynamics, the authors studied five infants with suspected NAHI in whom symptomatic bilateral mixed- or low-density subdural collections were revealed on imaging studies; the patients underwent burr hole evacuation of the hematoma and external drainage. Once decompression was achieved, radiotracer was injected into the lumbar subarachnoid space, and the subdural drainage system was monitored for appearance of the isotope. In all five cases, the radiotracer moved rapidly from the lumbar subarachnoid space into the convexity subdural space and then into the external drainage system. This indicated the possibility that some of these mixed-density subdural collections were acute blood mixed with CSF rather than acute-on-chronic collections arising from rebleeding subdural membranes. The authors propose that, during infancy, tears in the loosely adherent arachnoid envelope at the main arachnoid granulation site along the superior sagittal sinus may result in a considerable amount of CSF mixing with acute blood in the subdural space, creating a hematohygroma.