One hundred-fifty pneumoenecephalographic studies, during which serial anteroposterior tomography was performed, were reviewed to determine the appearance of the lateral recesses of the fourth ventricle in both normal and pathological states. Displacement of the lateral recesses will aid in differentiating between intra- and extra-axial posterior fossa tumors. In prepontomedullary tumors, a “trumpet-like” enlargement of the lateral recess orifice is seen. When cerebellar degeneration is present, change in size, shape, and position of the posterosuperior and lateral recesses may indicate fourth ventricular enlargement not otherwise suspected.