The diagnosis of transtentorial brain herniation has long relied on encephalography, then arteriography. Computerized tomography (CT) is a safer method which permits a more precise and earlier visualization of temporal and central herniations and herniation of the culmen cerebelli, which are the three varieties of transtentorial herniation. In an attempt to evaluate the reliability of CT images of herniation, the authors have conducted a study of anatomy-CT correlations, using autopsy specimens of brains with these three types of transtentorial herniation. Temporal herniation was well studied, irrespective of the CT reference plane. Direct visualization of temporal uncus herniation and filling of the homolateral perimesencephalic cistern was regularly obtained. Central herniation was better visualized when the occipito-temporal plane was used as reference. The disappearance of perimesencephalic cisterns on CT sections through the widest part of the tentorial incisura is the best element of diagnosis. Herniation of the culmen is easily studied on the conventional orbito-meatal plane. Provided CT scans are performed with the technique they recommend, the authors consider that this examination is reliable for the diagnosis of transtentorial herniation. Some variations in the anatomy of the incisura may explain why the clinical consequences of herniation are varied. CT perfectly shows the configuration of this notch and therefore may be helpful in predicting the prognosis.