We performed a comprehensive immunohistochemical evaluation of claudin 4 protein expression in paraffin-embedded tissue samples from 72 patients with primary infiltrating pancreatic cancer, 38 patients with metastatic pancreatic cancer, and a panel of normal control tissue samples from various organs. In 11 samples of primary infiltrating pancreatic cancer, foci of pancreatic intraepithelial neoplasia (PanIN) were present and also were analyzed for claudin 4 protein expression. Intense positive claudin 4 immunolabeling was noted within virtually all primary (71/72 [99%]) and metastatic (49/49 [100%]) pancreatic cancer tissue samples analyzed and in 10 of 11 samples of PanIN. In all cases, immunolabeling was noted in a membranous distribution. Claudin 4 protein also was detectable in normal breast, prostate, bladder, and gastrointestinal mucosa, although expression was substantially less intense than that seen in pancreatic cancer tissue samples. Our findings support the use of claudin 4 as a target for novel therapeutics or radioimaging of infiltrating pancreatic cancer. Furthermore, the finding of claudin 4 overexpression within pancreatic intraepithelial neoplasia, the precursor lesion of pancreatic cancer, suggests a potential benefit of imaging claudin 4 before the development of an invasive carcinoma.