We reviewed the unenhanced computer tomography (CT) scans of 53 patients with surgically proven acute cholecystitis, where 27 patients presented with hyperdense gallbladder wall. To our knowledge, this sign was never reported before. Because mucosa is highly sensitive to ischemia, early mucosal necrosis and hemorrhage may result in CT-detectable high density. Similar episode may also occur in acute cholecystitis. This sign also reflects high probability for acute gangrenous cholecystitis. We suggest that patients with this sign should have urgent treatment.