Neoplastic transformation of epithelial cell sis commonly associated with altered synthesis of mucin glycoproteins. Few studies have been performed on the correlation between MUC 1 expression and pancreatic carcinoma using immunohistochemical methods. We compared the patterns of MUC 1 expression in normal pancreatic tissue, in pancreatic carcinoma, and in chronic pancreatitis. Immunohistochemical studies were performed using 3 monoclonal anti-MUC 1 antibodies (12C10, 1G5, and H23) on surgical specimens and on fine-needle aspiration biopsy specimens. In the neoplastic cells from adenocarcinomas, high levels of cytoplasmic MUC 1 expression were observed, with some membrane staining. No such cytoplasmic expression was observed in normal tissue, tissue from chronic pancreatitis, or benign neoplastic tissue. These data show conspicuous quantitative and qualitative differences between the patterns of MUC 1 expression observed in nonmalignant vs malignant pancreatic tissue and may be useful in the histologic diagnosis of adenocarcinoma in biopsy samples.