CT scans in 49 patients with surgically proven subdural empyema were evaluated. The empyemas were crescentic or lentiform extra-axial hypodense collections (density approximating that of cerebrospinal fluid) with prominent, sharply etched medial rim enhancement. Enhancement of the adjacent cerebral cortex was identified in many cases. Mass effect was always present and in 10 cases so extensive that it overshadowed a small extra-axial collection. CT allowed for precise localization of the lesion, including contiguous or isolated involvement of the interhemispheric subdural space. Mortality was 12% (6/49 cases), a marked improvement when compared with mortality figures obtained prior to the advent of CT (40%). CT findings indicative of involvement of the adjacent parenchyma via retrograde thrombophlebitis with resultant infarction and/or abscess formation were associated with poor prognosis. Improvement in prognosis since the advent of CT is the direct result of early accurate diagnosis and timely intervention.