High-grade transformation is seen in about 10% of well-differentiated acinic cell carcinomas, usually in older patients with long-standing or recurrent tumors. It is often heralded by a period of rapid growth, fixation to surrounding tissues, pain, and facial paralysis. Rarely, high-grade morphology may be seen at the time of initial presentation. The tumor usually consists of areas of conventional (low-grade) acinic cell carcinoma with associated high-grade carcinoma. The high-grade areas are composed of undifferentiated or poorly-differentiated cells with solid or trabecular growth pattern (as shown here). Increased mitotic activity (including atypical mitotic figures) and necrosis may be present. The prognosis is usually poor.