Subdural hematomas, whose absorption values approximate those of adjacent brain, are not visualized in routine computed tomography. Two clues indicating the presence of such "isodense" subdural hematomas are: (1) unilateral effacement of cerebral sulci on the convexities, and (2) midline shift or mass effect on the ventricles in the absence of abnormal areas of diminished or increased density in the brain. Nine cases were detected on pre- and postcontrast studies in 2,500 CT scans of the brain over a 10 month period. Delayed CT scanning 4-6 hr after intravenous contrast injection showed enhancement of the subdural hematoma in three of seven cases.