The most frequent encephalographic abnormality in children, and perhaps also in adults, is enlargement of the lateral ventricles. My associates and I have felt the need of a quantitative expression to describe more accurately the degree of enlargement and to define with some precision, if possible, the normal limits of ventricular size. It is assumed, in spite of a few scattered observations reported in the literature to the contrary, that the cerebral ventricles are of a relatively fixed size and shape and, in the absence of disease, do not vary significantly from day to day. Further systematic study is desirable to establish this point. The simplest measurement to make, and at the same time one of the most significant, is the transverse diameter of the anterior horns on a film exposed in the anteroposterior projection with the posterior part of the skull down. This allows filling of the anterior horns