A series of 3,500 cases examined by computed tomography (CT) of the head were reviewed to identify patients who had symptoms and signs suggestive of neurologic disease in the posterior fossa. Three hundred forty eight such cases (about 10%) were found, and the scans were reviewed to determine the character of lesions that could be detected and the accuracy of the neuroradiologic diagnosis. Overall, 16 errors were made (4.6%), of which 13 were false negatives and 3 were false positives. False negatives included nine tumors, two infarcts, and two arteriovenous malformations. Four of the tumors and the two arteriovenous malformations were small and caused no visible alterations in density even when analyzed retrospectively. In the remaining seven cases, specific findings either were overlooked or were obscured by artifacts, and, therefore, the diagnosis was not made. The three false positive diagnoses were made through a misinterpretation of density changes that were artifactual. No changes in the intracranial absorption coefficients were detected in 109 cases. Abnormal changes were evident in 226 cases in which structural lesions were subsequently found. Many of the lesions had a characteristic appearance, but in certain clinical situations, which include searches for metastatic disease, acoustic neuromas, meningiomas aneurysms of the basilar artery, and arteriovenous malformations, the injection of intravenous contrast medium was helpful, or even essential, for diagnostic success. The prominence of clinical signs and symptoms is not always an accurate index to the presence of structural alteration of the posterior fossa. We believe, therefore, that this portion of the cranium should be included in every CT study.