The size and shape of the lateral ventricle are frequently altered by intracranial lesions, and this may bereflected on cerebral angiogram. The size and dilatation of the lateral ventricle may be estimated by the courseof the thalamostriate vein (TSV) and the distance between the midline and the TSV in frontal projection, thecourse of the pericallosal artery and the distance between the venous angle and subependymal veins in lateralprojection. However, little description can be found in the literature about the method of expressing the size anddegree of dilatation of the lateral ventricle on cerebral angiogram. The authors have attemptedto find out an easyway of precisely estimating the size of the lateral ventricle and to observe how it can be applied in the patientswith various expanding intracranial lesions. We measured the angle formed between the internal cerebral vein (ICV)and the TSV in the anteroposterior roentgenograms of venous phase in normal group composed of 61 patients in whomno significant abnormality could be detected neurologically or by other methods, and in 18 patients with expandingintracranial lesions. The results obtained are as follows; 1. In the normal group, the average angle formedbetween the ICV and TSV on the anteroposterior angiogram obtained with the central beam projected making an angleof 10 to 15degrees with the orbitomeatal line was 25.7±3.9degrees, ranging from 19 to 34degrees. The anglemeasured from 20 to 30degrees in 85% of the normal group. There was no significant difference between the male andthe female as well as between the children and adults. 2. The measurement of the angle was found to reflecxtfaithfully the size of the lateral ventircle on the side examined, increasing as the lateral ventricle dilated.When the angle measures more than 33degrees, the lateral ventricle would certainly be dilated. The lateralventricle can be taken as moderatly delated when the measurement exceeds 40degrees and as severely dilated whenthe angle is over 50degrees. On the contrary, it may be conceived that lateral ventricle is compressed anddeformed by an expanding lesion on the same side when the angle measures less than 20degress. 3. The measurementof the angle varied even in the same individual as the angle of central beam to the orbitomeatal line changed. Itmeasured larger on the straight anteroposterior view and smaller on the half-axial view. It is postulated that thenormal value given above can only be applied when the anteroposterior roentgenogram is obtained in standardprojection, in which the bifurcation of the internal carotid artery is usually thrown well above the orbit and thepetrous bone. 4. The main axis of direction of the TSV was stright in normal group and deviated laterallydescribing downward and lateral convexity when the lateral ventricle was dilated.