MR images of 24 patients with 33 subdural collections were retrospectively reviewed to determine the spectrum of MR findings associated with such lesions. The lesions were dated by history, when available. Hematomas were grouped as follows: acute, four; early subacute, four; late subacute, four; chronic, 13. Six collections were classified as rehemorrhage; and two patients had CSF hygromas. Subdural hematomas evolved in a pattern similar to intracerebral hemorrhage with the exception of chronic subdural hematomas, in which isointensity of hypointensity relative to gray matter was observed on short TR/TE images compared with the persistent very high signal intensity noted in chronic parenchymal hematomas. Hemosiderin was rarely seen in chronic hematomas. These findings are most likely the result of the absence of a blood-brain barrier, which allowed clearance and dilution of blood products. Subdural hematomas with repeat hemorrhage demonstrated multiple phases of bleeding with layering phenomenon and more frequent hemosiderin deposition. It is possible that the clearance of blood products, as observed in chronic subdural hematomas, is impaired or poorly functional when rehemorrhage occurs. The persistence of high signal from methemoglobin in a hematoma that is expected to be in the chronic phase also suggests repeated hemorrhage. Acute CSF subdural hygromas had signal intensities identical to CSF without MR evidence of blood products. At surgery, clear fluid under pressure was found. MR imaging, with its unique ability to delineate the various phases of hemorrhage, is well suited to the evaluation of subdural hemorrhage.