High-grade transformation of adenoid cystic carcinoma (ACC) (previously referred to as dedifferentiation) is a rare phenomenon that does not fit into the traditional ACC grading schemes. The importance and minimal criteria for distinction from solid (grade III) ACC are not well established. We report 11 new cases and review the literature to further define the profile of this tumor. The median age was 61 years (range: 32 to 72 y) with a male predominance (male to female ratio of 1.75:1). The most commonly involved sites were sinonasal (4/11) and submandibular (4/11). Lymph nodes were pathologically positive in 4/7 (57.1%) cases. Distant metastases to the lung (n=2) and soft tissue of the shoulder (n=1) were observed. Five of 9 patients (55.6%) died, all within 5 years with a median overall survival of 12 months. Histologically, ACC with high-grade transformation was distinguished from conventional ACC by nuclear enlargement and irregularity, higher mitotic counts, and the loss of the biphasic ductal-myoepithelial differentiation. Useful supportive criteria were prominent comedonecrosis and fibrocellular desmoplasia. The most common morphologies for the high-grade component were poorly differentiated cribriform adenocarcinoma and solid undifferentiated carcinoma. Micropapillary and squamoid patterns were occasionally present. Ki-67 and p53 labeling indices were elevated in the high-grade components, though c-kit and cyclin-D1 were not. ACC-high-grade transformation is a highly aggressive salivary gland tumor with a variety of histologic patterns. The high propensity for lymph node metastases suggests a role for neck dissection in patients with this rare tumor.