Although computed tomography (CT) remains the most frequently used imaging examination in acute cerebral infarction, its sensitivity for early detection of strokes remains limited. In middle cerebral artery (MCA) strokes less than 6 hours old, loss of definition of the gray-white interface in the lateral margins of the insula ("insular ribbon") was observed. The acute and subacute CT findings in 11 retrospective and 16 prospective patients are presented. Loss of the insular ribbon was detected in 12 of the prospective cases and in all 11 retrospective cases. The insular ribbon is supplied by the insular segment of the MCA and its claustral branches. With cessation of MCA flow, the insular ribbon becomes the region most distal from the anterior and posterior cerebral collateral circulations. Consequently, the insular ribbon effectively becomes a watershed arterial zone. Loss of the insular ribbon is thus a reflection of acute edema due to infarction. Loss of the insular ribbon appears to be another frequent and reliable finding in acute MCA stroke.