MUTISM AND A variety of other neurobehavioral symptoms have been reported anecdotally after the removal of posterior fossa mass lesions. To determine the incidence and clinical spectrum of this syndrome, a detailed review was performed of patients undergoing resection of infratentorial tumors at our institution during the last 9 years; 12 of 142 patients (8.5%) manifested this syndrome, the largest series of such patients reported to date. Each child had a lesion that involved the vermis; seven had medulloblastomas, three had astrocytomas, and two had ependymomas. The incidence among children with vermian neoplasms was 13%. Ten children underwent division of the inferior vermis during tumor resection, and three had a superior vermian incision; one child underwent both superior and inferior vermian incisions. In 10 children, mutism developed in a delayed fashion postoperatively. The speech disturbance was associated with poor oral intake in 9 children, urinary retention in 5, long-tract signs in 6, and bizarre personality changes, emotional lability, and/or decreased initiation of voluntary movements in all 12. Neuropsychiatric testing, performed in seven children, confirmed impairments not only in speech but also in initiation of other motor activities. Ten children regained normal speech, bladder control, and neurological functioning, other than ataxia and mild dysarthria, within 1 to 16 weeks; two children had significant residual deficits. Characteristically, affect and oral intake returned to their preoperative baseline before the speech difficulties began to resolve. A detailed radiological review of these cases in parallel with 24 cases of vermian tumors without mutism identified only one factor that was significantly associated with the mutism syndrome, bilateral edema within the brachium pontis (P < 0.01). Neither the size of the tumor nor the length of vermian incision was associated with the development of mutism. The clinical features of this syndrome in the context of these imaging findings suggest that the mutism syndrome results from transient impairment of the afferent and/or efferent pathways of the dendate nuclei that are involved in initiating complex volitional movements. The clinical courses of our patients are presented and compared with those of similar cases in the literature in an attempt to evaluate the validity of this hypothesis.