Pleomorphic adenoma (PA) is known to occasionally progress to carcinoma, but the mechanisms of its malignant transformation have not been fully elucidated. S100P, an EF-hand calcium-binding protein, has recently been proposed as an initiator of carcinogenesis in some kinds of epithelial tumors. In this study, we aimed to elucidate the potential role of S100P in tumorigenesis and stepwise progression of carcinoma ex pleomorphic adenoma (CXPA) with ductal differentiation. In 31 ductal type CXPAs (8 in situ, 5 intracapsular, and 18 extracapsular) and 28 PAs (21 conventional and 7 atypical) of the salivary gland, we examined the protein expression of S100P, androgen receptor (AR), HER2/neu, p53, and Ki-67 by immunohistochemistry. HER2 expression, p53 expression, and the Ki-67 labeling index were higher in CXPAs than in atypical PAs and conventional PAs, whereas the AR expression level was relatively high even in atypical PAs. S100P overexpression was significantly more prevalent in CXPAs (27 cases; 87.1%) than in atypical PAs (2 cases; 28.6%) and conventional PAs (1 case; 4.8%) (P<0.05). High prevalence of S100P expression was observed in each intraductal, extraductal-intracapsular, and extracapsular component of CXPAs. In addition, equivalent, high-level S100P expression was observed in all histologic subtypes of the malignant component of CXPAs. These results indicate that S100P may play an important role in malignant transformation of ductal cells of PA, and that immunohistochemical staining for S100P would be a useful diagnostic marker for identifying the early phase of CXPA, in combination with AR, HER2, p53, and Ki-67.