A 19-year-old boy suffered from headache and intermittent CSF rhinorrhea, was admitted to Matsuyama Shimin Hospital on June 1, 1979. Two months prior to admission he had had a frontal head injury with confusion and the right nasal bleeding. Plain skully x-ray films and biplane (axial & coronal) CT revealed intracerebral pneumocephalus in the right frontal lobe with depressed basal skull fracture into the right ethmoid sinus. Clinical conservative courses of intermittent CSF rhinorrhea, headache and vomiting were related to the changes of the air shadow on plain skull films. Preoperative metrizamide CT Cisternography was done on July 5. Sequential CT cisternograms demonstrated ventricular reflux at 1 hr and partial obstruction of the basal cistern and supratentorial subarachnoid space at 3 hr, which attributed to the mass effect of the air cysts. They also demonstrated an interesting finding, the accumulation of metrizamide into the intracerebral air cyst at 3 hr, suggesting transependymal penetration of contrast medium. At 24 hr, the air cyst decreased in size and the metrizamide disappeared. CT cisternograms 8 months after the surgery showed no evidence of air cyst but remained a low dense porencephalic cyst. Ventricular reflux was seen at 3 and 6 hr but there was no accumulation of metrizamide into the cyst at any hr. Filling patterns of the basal cistern and supratentorial subarachnoid space returned normal except the defect in the anterior interhemispheric cistern. Transependymal penetration of metrizamide in this case can be explained by the mechanism of pressure gradient between the ventricle and the air cyst. Thus the postoperative CT cisternograms showed no penetration because of the absence of pressure gradient, whereas the ventricular reflux and the same ependymal septum still remained. No mention has been made about the CSF flow dynamics in intracerebral pneumocephalus and their relation to the CSF rhinorrhea in previous literature. Intermittent CSF rhinorrhea of this case will attribute to the CSF accumulated in the air cyst as mentioned above.