The effect of decompressive craniectomy on regional cerebral blood flow (rCBF) was investigated in five patients with severe head trauma who underwent decompressive craniectomy. Repeated rCBF studies using single photon emission computed tomography with ^99mtechnetium-hexamethyl-propyleneamine oxime observed that a hyperperfusion area (focal CBF increase) occurred in the decompressed brain within 24 hours after decompressive craniectomy. The hyperperfusion area in the decompressed brain enlarged and increased in severity by 1 week after surgery. However, it attenuated and disappeared by 1 month after surgery. The chronology of the hyperperfusion area corresponded to the change in the swelling of decompressed brain observed by x-ray computed tomography. Patient consciousness showed a significant and progressive improvement in the postoperative 1 month period. Decompressive craniectomy may cause a focal CBF increase in the decompressed brain related to the beneficial effect in patients with acute severe head trauma.