The epithelial expression of apomucins MUC1, MUC2, MUC3, and MUC5/6 was examined in normal pancreas and in pancreatic lesions, using immunohistochemical methods. In normal pancreas (n = 5), MUC1 apomucin was expressed in ducts and some acini, but there was no expression of MUC2, MUC3, or MUC5/6. In chronic pancreatitis (n = 5), MUC1 apomucin was expressed, but expression of the other apomucins was not noted. However, mucous hyperplastic foci of pancreatic ducts expressed MUC5/6 apomucin in 2/5 cases (40 percent). In intraductal papillary-mucinous neoplasm (IPMN) of the pancreas (n = 9), MUC1, MUC2, MUC3, and MUC5/6 apomucins were expressed in 8/9 (89 percent), 0/9 (0 per cent), 4/9 (44 per cent), and 9/9 (100 per cent) case, respectively. In pancreatic mucinous cystadenoma (n = 8), MUC1, MUC2, MUC3, and MUC5/6 apomucins were expressed in 7/8 (88 percent), 0/8 (0 percent), (25 percent), and 3/8 (38 percent) cases, respectively. In invasive ductal adenocarcinoma of the pancreas (n = 25), expression of MUC1, MUC2, MUC3, and MUC5/6 apomucins was found in 25/25 (100 percent), 1/25 (4 percent), 20/25 (80 percent), and 24/25 (96 percent) cases, respectively. Atypical mucous duct hyperplasia near cancer cells consistently expressed MUC1 apomucin and occasionally expressed MUC3 and MUC5/6. In positive cases, MUC1 apomucin expression was noted in the cell membrane facing the ductal or neoplastic lumina, while expression of MUC2, MUC3, and MUC5/6 apomucins was found in the cytoplasm. These results suggest that MUC3 and MUC5/6 apomucins newly emerge during the neoplastic transformation of pancreatic mucinous cystadenoma and IPMN and during pancreatic ductal carcinogenesis, while MUC1 apomucin remains positive and MUC2 apomucin remains almost negative during neoplastic transformation.