Two pancreatic acinar cell carcinomas with distinctive morphologic features are described. Both tumors showed a variable combination of different histomorphologic patterns formed by tumor cells in different degrees of acinar differentiation. They caused diagnostic problems involving differential diagnosis from normal pancreas and from mixed acinar-endocrine tumor. The presence of mitosis, nuclear atypia, and tumor necrosis are helpful for recognizing its neoplastic nature, and the ultrastructural finding of zymogen-like but not neurosecretory granules prevented the misdiagnosis of a mixed acinar-endocrine tumor. Immunohistochemical studies revealed variable immunostaining patterns corresponding to different types of tumor cell present. Both tumors were found to be aneuploid by flow cytometric DNA study. Our study confirmed previous observations that pancreatic acinar cell carcinomas are highly malignant tumors in spite of the fact that they can differentiate into highly differentiated acinar cells.