Acinar cell carcinoma (ACC) of the pancreas is very rare, which usually grows expansively. Recently, a variant of ACC with predominant growth in the pancreatic ducts has been proposed, and is speculated to have potentially less aggressive behavior. The aim of this study was to investigate how the pancreatic duct system is related to the growth and extension of ACC. We reviewed the detailed gross and histologic features of 13 cases of ACC, of which 7 (54%) showed intraductal polypoid growth (IPG) of the tumor in the large pancreatic ducts with a mean IPG length of 24.8 mm. Tumors with IPG were found to spread characteristically along the pancreatic ducts as extending polypoid projections, filling the ducts and destroying the duct walls, although tumors did not tend to extend beyond the pancreatic parenchyma. Comparison of the clinicopathologic characteristics showed that ACC with IPG had less infiltrative features including lymphatic, venous, and neural invasion, formation of tumor thrombus in the portal vein, nodal metastasis, and invasion beyond the pancreas to the surrounding organs; death in only 1 case (14%) of ACC with IPG was the result of ACC itself. In contrast, ACC without IPG frequently showed more infiltrative growth, and was the cause of death in 50% of patients with this type of tumor. Intraductal dissemination of ACC in pancreatic ducts was proven in 1 case of ACC with IPG. These findings suggest that a significant proportion of ACC shows IPG, which is potentially linked to less aggressive clinicopathologic characteristics.