OBJECTIVEIntracerebral hemorrhage occurring after removal of a chronic subdural hematoma (CSH) is a rare but usually devastating postoperative complication. In patients with CSH, we determined cerebral blood flow immediately after surgical decompression to clarify the pathogenic mechanism of this complication.METHODSIn 22 patients with unilateral CSH, a catheter was inserted into the hematoma cavity through a single burr hole without leakage of the contents. Cerebral blood flow was measured using single-photon emission computed tomography 1 day before surgery, immediately after rapid decompression by opening the catheter, and 3 days after surgery.RESULTSSingle-photon emission computed tomography imaging immediately after decompression demonstrated areas of hyperemia in nine patients (41%). In all nine patients, hyperemia was observed in the cortex beneath the CSH, and it disappeared on the 3rd postoperative day. The patients with hyperemia were significantly older than those without hyperemia. Other variables, including preoperative cerebral blood flow, intrahematoma pressure before decompression, and mean arterial blood pressure during decompression had no significant effect on the occurrence of hyperemia.CONCLUSIONIn elderly patients, rapid decompression of CSH frequently results in transient hyperemia in the cerebral cortex beneath the hematoma.