ObjectiveTo examine the relationship between histopathology, immunohistochemistry, and clinical behavior in atypical and low-grade malignant vestibular schwannomas.Study DesignThe study design was a retrospective case review in conjunction with a histopathologic and immunohistochemical proliferation marker study of archival specimens.Data SourcesA tertiary referral center's anatomic pathology and vestibular schwannoma computerized databases.MethodsThe diagnosis of atypical or low-grade malignant vestibular schwannoma was based on the number of mitotic figures present per tumor slide. MIB1 labeling indices were used to compare the proliferative activity of the atypical and low-grade malignant groups with that in an age-matched and size-matched control group.ResultsEight cases of atypical and six cases of low-grade malignant vestibular schwannoma were diagnosed from 1990 to 1998. In the atypical and low-grade malignant groups, respectively, the average patient age was 54.3 years (range, 38–74 yr) and 50 years (range, 38–72 yr), and the average total tumor size was 1.53 cm (range, 0.7–3.5 cm) and 1.55 cm (range, 1.5–2 cm). Two recurrences were identified from the low-grade malignant group, and there was one postoperative House-Brackmann Grade III facial weakness. There were no recurrences or facial palsies in the atypical group. No distant metastasis or aggressive local invasion was observed in either group. MIB1 labeling indices were significantly (p ≤ 0.001) higher in the atypical (4.69%) and low-grade malignant (5.23%) groups than in the control group (1.99%).ConclusionsThese findings suggest a tendency for recurrence in proliferative tumors; however, the designation of malignancy should be reconsidered.