151 Background: We recently reported that a serum-based enzyme immunoassay employing the PAM4 antibody was able to correctly identify 82% of patients with known PDAC and, importantly, that this assay had promising sensitivity for detecting early-stage disease. We now extend these findings in a much larger patient population that includes over 600 sera from both malignant and benign diseases of the pancreas and surrounding tissues. Methods: In a blinded analysis, sera from patients with confirmed PDAC (N=298), other cancers (N=99), benign disease of the pancreas (N=126), and healthy adults (N=79) were evaluated by enzyme immunoassay for concentration of PAM4-antigen levels. Results: Overall sensitivity for detection of PDAC was 76%, with 64% of stage-1 patients testing positive and a higher sensitivity (85%) for advanced disease. For the most part, sera from patients with neuroendocrine tumors of the pancreas or cancers of other origin (squamous, GIST, etc.) did not have elevated levels of the PAM4-antigen. Approximately half of the patients with ampullary (48%) and extrahepatic biliary (50%) adenocarcinomas had positive levels of circulating PAM4-antigen. Of 126 patients diagnosed with benign conditions of the pancreas, only 24 (19%) were positive and, in particular, 18 of 80 (23%) patients with chronic pancreatitis (CP) were positive. ROC curve analysis demonstrated a statistically significant difference between the PDAC and CP groups (P < 0.0001), with an area under the curve of 0.84 ± 0.02 (95% CI: 0.79 – 0.89). The positive- and negative-likelihood ratios for differentiating PDAC from benign conditions of the pancreas were 4.00 and 0.30, respectively. Conclusions: The PAM4-immunoassay detects nearly two-thirds of stage-1 PDAC patients, and does so with high discriminatory power with respect to benign pancreatic disease. Our results provide a rationale for longitudinal surveillance of patients considered at high-risk for PDAC (e.g., familial pancreatic cancer, new-onset diabetes, etc.) with the PAM4 assay. (Supported in part by NIH grants CA096924, CA120432 and CA62924, and the Turpin Foundation.)