Intraoperative ultrasound (US) was compared to computed tomography (CT) in 41 intracranial and 6 spinal cord tumors. The studies correlated closely except for primary gliomas. Eight of the 22 primary intracranial gliomas (37%), including 1 low-grade and 7 anaplastic tumors, were larger and more extensive on US than on CT. Margins of non-enhanced primary astrocytomas were shown by US but not CT. Four anaplastic tumors (19%) exhibited echogenicity extending beyond the enhanced area. In 4 patients an enhanced lesion contained a lucent center which proved to be echogenic. Low-grade astrocytomas were relatively homogeneous on US, while anaplastic astrocytomas were more inhomogeneous. Cysts could be found in both types of astrocytomas and were often small and multiple. The echo pattern was not helpful in differentiating metastases from primary tumors, although all of them had sharp margins. Sonography of the central nervous system can provide valuable information about tumor morphology and margins.