Adenomyoma and adenomyomatous hyperplasia of the Vaterian system are consistently benign lesions. Clinically, adenomyoma mimics frequently ampullary adenoma or carcinoma, and biopsy analysis is often difficult. The histogenesis of ampullary adenomyoma and adenomyomatous hyperplasia is still subject to debate. We present a retrospective study of clinicopathological features of 13 cases of surgically resected ampullary adenomyoma. The age of our patients was between 38 and 78 years (mean: 63 y). The preoperative diagnosis was ampullary tumor or tumor of the head of the pancreas. On macroscopy, a white, firm lesion of the ampullary wall was observed; its size ranged between 10 and 30 mm. Histologically the lesion consisted of multiple glandular structures surrounded by a fibroblastic/myofibroblastic proliferation, resulting in a "pseudo-hypertrophy" of the Vaterian system. The immunophenotype of the epithelial component was cytokeratin 7+/cytokeratin 20-, similar to that of the normal biliary and pancreatic duct system. The epithelial cells exhibited low proliferative activity. The hyperplastic myofibroblastic cells expressed smooth muscle actin. A complete pancreatic heterotopy contiguous with the adenomyoma was noted in three cases. Adenomyoma and adenomyomatous hyperplasia of the Vaterian system are benign lesions frequently treated by extensive surgery because of long-term biliary obstruction. The clinicopathological characteristics suggest either a reactive and/or a malformative, nonneoplastic nature for this lesion, which could, in some cases, develop from heterotopic pancreas. The immunophenotype of epithelial cells may be a useful tool for differentiating it from ampullary adenoma on biopsy specimens.