OBJECTIVE Cerebellar mutism is a common complication of posterior fossa tumor resection. We observed marked, preoperative brainstem compression on MR imaging, among patients who developed postoperative mutism. This study was designed to investigate if an association was indeed present. METHODS Patients (18 months-18 years) undergoing resection of a midline, posterior fossa tumor were retrospectively reviewed. Demographic data, tumor pathology, mutism onset and duration, and postoperative complications were obtained from hospital records. Pre- and postoperative MR images were studied to assess tumor size and the severity of pons compression (an estimate of the mechanical and distortional forces imparted by the tumor). RESULTS Patients with mutism showed greater preoperative pons compression and a greater increase in postoperative pons diameter. CONCLUSIONS We predict that brainstem compression may represent white-matter injury from (1) surgical manipulation and traction, and (2) axonal damage caused by the release of the tumor's compressive force and ensuing axon distortion and dysfunction. The results provide support that mutism may be largely caused by white-matter damage disrupted axon integrity and function.