Rapid neurological alteration was observed in association with concave deformity of the skin flap following a large decompressive craniectomy. In an erect position, hemiparesis and dysphasia showed marked deterioration with a sinking skin flap and CT-scan showed considerable distortion of the underlying brain tissue. Those changes were rapidly improved by restoration of normal curvature of the skin flap in a supine position. EEG findings coincided with such rapid alteration. Neurological changes may be due solely to concave deformity of the skin flap and the underlying brain tissue; this characteristic phenomenon might be described as “the syndrome of the sinking skin flap.” The presence of shunt system seems to play an important role in appearance of such a phenomenon.