The authors describe the case of a 57-year-old woman who had a right-sided hearing disturbance that had remained untreated for 1 year. The diagnosis was of a right cerebellopontine angle tumor, and the patient underwent its removal via retrosigmoid approach. Pathologically, the tumor was a typical benign neuroma. Growth of residual tumor was detected 4 years after the initial operation, and it was treated with gamma knife surgery (GKS). Six months later, the tumor had grown, and the patient underwent surgery via a combined retrosigmoid-translabyrinthine approach. Abnormal mitotic figures were observed on histological studies, indicating that the tumor had become malignant. Thereafter, the tumor grew rapidly, and the patient died 6.5 years after the initial treatment. It cannot be ruled out that GKS affected the outcome, but the causal sequence was unclear. Because such a patient is rare, documentation of the case was considered clinically important.