Extra-axial posterior fossa masses [in man] could be diagnosed reliably by computed tomography (CT) in most cases. Acoustic and trigeminal neurinomas, meningiomas, cholesteatomas and other extra-axial masses could usually be distinguished from intra-axial masses by asymmetric widening of the basal subarachnoid spaces, bone destruction, continuity of the tumor mass with the tentorium or bone and more sharply defined margins. Multiple-cut study of the posterior fossa improved visualization of the 4th ventricle and basal cisterns. Interpretation of cisternal changes in association with changes in the 4th ventricle and abnormal attenuation coefficients permits accurate diagnosis of posterior fossa masses.