Materials and methods: We designed a prospective study to investigate single photon emission tomography (SPECT) findings in patients with or without mutism who had undergone posterior fossa surgery. These patients had a Tc-99m-hexamethylpropyleneamine oxime-SPECT scan postoperatively. SPECT was performed between the 1st and. 2nd week after surgery. The patients with cerebellar mutism had a second SPECT scan after regaining their power of speech. The patients were evaluated in terms of age, gender, preoperative and postoperative neurological and radiological findings, tumor size and location, vermian incision and histopathology. Results: Six girls and 5 boys, ranging in age from 3 to I I years who had undergone posterior fossa tumor surgery were included in this study as controls. Their tumors were located in the vermis and fourth ventricle in 8 patients, in the cerebellar hemisphere in 1, fourth ventricle in 1, and I patient had a fourth ventricular tumor extending the cerebello-pontine angle. The histopathology of the tumors was as follows: astrocytoma in 5 patients, ependymoma in 3, medulloblastoma in 1, dermoid tumor in 1, and choroid plexus papilloma in I patient. Postoperative cerebellar mutism developed in 2 girls and I boy whose ages ranged from 2 to 5 years. Pathological examination revealed 2 medulloblastomas and I ependymoma. Mutism developed 2 to 4 days after surgery. All of the patients with cerebellar mutism showed either clinical or radiological signs and symptoms of brain stem involvement. SPECT findings were similar in the patients with cerebellar mutism and in those without. Conclusions: SPECT findings are not specific enough to explain how cerebellar mutism occurs. Clinical signs of brain stem involvement seem to be a significant risk factor in the development of cerebellar mutism.