The size and shape of the lateral ventricle are frequently altered by intracranial lesions, and this may be reflected on cerebral angiogram. The size and dilatation of the lateral ventricle may be estimated by the course of the thalamostirate vein (TSV) and the distance between the midline and the TSV in frontal projection, the course of the pericallosal artery and the distance between the venous angle and subependymal veins in lateral projection. However, little description can be found in the literature about the method of expressing the size and degree of dilatation of the lateral ventricle on cerebral angiogram. The authors have attempted to find out an easy way of precisely estimating the size of the lateral ventricle and to observe ow it can be applied in the patients with 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 whom no significant abnormality could be detected neurologically or by other methods, and in 18 patients with expanding intracranial lesions. The results obtained are as follows: