The expression of MUC-1 mucin (membrane-associated mucin) and MUC-2 mucin (secretory mucin) were immunohistochemically examined in 46 invasive ductal carcinomas (IDC) and 16 intraductal papillary mucinous tumors (IPMT) of the pancreas. Intraductal papillary mucinous tumors usually reveal expansive growth. However, of the 16 IPMT examined in the present study, three showed an invasive growth pattern, which was similar to 'mucinous carcinoma', around the non-invasive growth areas. Of 46 IDC, MUC-1 mucin detected by monoclonal antibodies, DF3 and MY.1E12, was expressed in 44 cases (96%) and in 45 cases (98%), respectively, whereas MUC-2 mucin detected by polyclonal antibody, anti-MRP, was not expressed in any of the cases (0%). In contrast, in the non-invasive growth areas of the 16 IPMT, MUC-1 mucin detected by DF3 and MY.1E12 was expressed in four cases (25%) and in six cases (38%), respectively, whereas MUC-2 mucin detected by anti-MRP was expressed in 13 cases (81%). The invasive growth areas of the three IPMT showed positive expression of MUC-1 mucins detected by DF3 and MY.1E12, although the non-invasive growth areas showed negative expression of MUC-1 mucins, except for their focal positive expression in one of the three cases. These findings indicate that the invasive growth areas of IPMT acquire a characteristic of MUC-1 mucin expression that is usually seen in IDC.