Maspin (SERPINB5), a serine proteinase inhibitor, was first identified as a potential tumor suppressor on the basis of its differential expression between normal mammary epithelial cells and human breast carcinoma cell lines. Recent studies have shown that maspin might be a prognostic tumor marker. Pancreatic ductal adenocarcinoma acquires maspin expression through hypomethylation of the maspin promoter. However, no study has investigated the prognostic significance of maspin expression in pancreatic ductal adenocarcinomas. In this study, we investigated maspin protein expression in a large series of 223 surgically resected pancreatic ductal adenocarcinomas using immunohistochemical staining and high throughput tissue microarrays. Maspin expression was correlated with postoperative survival and other clinicopathologic factors. Maspin was detected in 209 of these 223 (94% cases) pancreatic ductal adenocarcinomas including 39 (18% cases) focal (5-50% tumor cells) and 170 (76% cases) diffuse (>50% tumor cells). Fourteen (or 6% cases) pancreatic ductal adenocarcinomas did not show maspin expression by immunohistochemical staining (<5% tumor cells). Normal ductal epithelium is not labeled with maspin. Overexpression of maspin in pancreatic ductal adenocarcinoma is associated with worse postoperative survival especially in patients whose tumors exhibit diffuse expression of maspin. After adjusting other clinicopathologic factors, maspin expression remains to be an independent adverse prognosticator for postoperative survival. Maspin expression is not associated with patient age, gender, tumor size, tumor pathologic stage, lymph node status, and vascular invasion or perineural invasion. Nuclear labeling of maspin is associated with better tumor differentiation although this staining pattern is not associated with a better prognosis. In addition, maspin overexpression is also observed in 48% low-grade (grades 1a and 1b) pancreatic intraepithelial neoplasias (PanINs) and 78% high-grade (grades 2 and 3) PanINs, suggesting that maspin upregulation occurs early during the multi-step progression model of pancreatic ductal adenocarcinoma.