Subdural tension pneumocephalus (TP) following surgery for chronic subdural hematoma was analyzed in 5 cases from clinical standpoints of view, especially from CT findings. The cases were compared with 14 cases of subdural asymptomatic pneumocephalus (AP). The TP tends to complicate elderly patients who showed a poor re-expansion of the brain after irrigation of bilateral chronic subdural hematoma. In such patients, it is difficult to make an exact diagnosis of TP because of residual mass effect of the evacuated hematoma. Significance of mass effect caused by subdural air has been discussed in the literature only little. In this study, we found two new CT findings suggesting increased tension of subdural air. First, the subdural tensive air separates and compresses the frontal lobes. The compressed frontal lobes with widened interhemispheric space between the frontal poles mimic the silhouette of Mt. Fuji. We called this CT finding "Mt. Fuji" sign. The presence of air between the frontal poles associated with massive air over the frontal lobes presumably indicates an increased tension of the subdural air. "Mt. Fuji" sign was seen in 4 cases out of 5 TP cases. Another sign is the presence of multiple small air bubbles in the subarachnoid space, especially in the cisterns. We proposed that these air bubbles were trapped in the subarachnoid space through a tear of the arachnoid membrane which is caused by increased tension of air in the subdural space. This finding was present in 4 cases with TP. We emphasize that these two CT signs are helpful to make an accurate diagnosis of TP following surgery for chronic subdural hematoma.