Background: Distinguishing benign from potentially malignant PCL is problematic. Limited data support the role of EUS and FNA in these settings. Aim: Review and develop specific EUS characteristics that help to differentiate benign from potentially malignant PCL. Methods: Over a 24 month period, 39 consecutive patients (M/F 22/17; mean age 60, range 18- 79 y.) with known or suspected PCL based on CT and/or transabdominal ultrasound underwent EUS. FNA of PCL was performed in 20 patients with analysis for cytology, CEA, CA 19-9, mucin and/or amylase. 55% of patients received prophylactic antibiotics. For comparison purposes, benign cysts (simple cyst: SC, pseudocyst: PC, serous cystadenoma: SCA) were discriminated from those with malignant potential (mucinous cystadenoma: MCA, intraductal papillary mucinous tumor: IPMT, cystic islet cell tumor: ISLT, cystic adenocarcinoma: ACA based on the following EUS criteria (size, wall thickness, septation, presence of mass/papillary growth, and communication with main pancreatic duct). Results: 15 patients underwent surgery allowing histopathology correlation (SCA: 1, SC: 1, PC: 1, chronic pancreatitis: 1, choledochocele: 1, MCA: 3, IPMT: 1, MCA with carcinoma: 2, ISLT: 3, ACA: 1). Based on EUS, 3 patients were classified as benign and 12 as suspicious for potential malignancy. In patients with histopathology correlation, EUS accurately predicted the potential for malignancy or not in 66% (10/15) of patients. PPV of EUS for detecting suspicious cysts was 75% (9/12). All 4 patients with islet cell tumor or ACA who underwent FNA were correctly classified with EUS alone. In 5 patients incorrectly classified by EUS, results of cytology and CEA fluid analysis did not correct this misinterpretation. 4 patients with suspicious EUS appearance and benign pathology had cystic fluid CEA levels >150. One patient with benign EUS appearance and MCA on pathology had cystic fluid CEA level