Introduction: CA 19-9 has been identified as a derivative of sialic Lewis blood group A and is expressed in 95% of the population. Several studies have documented an overproduction of CA 19-9 in malignant pancreatic and biliary tree diseases. The objective of this study is to determine the accuracy of the tumor marker CA 19-9 differentiating benign and malignant bilio-pancreatic diseases. Material and Methods: Diagnostic test study. We reviewed the records of all patients with malignant bilio-pancreatic diseases and benign biliary calculous diseases evaluated in Hospital Base Osorno between August 2007 and December 2011, with CA 19-9 as part of their study. Results: 71 patients met the inclusion criteria, 17 men and 54 women, with a mean age of 60.7 +/- 15.3 years old. Twenty nine (40.8%) cases were benign and 42 (59.2%) cases malignant. For a cutoff level of 37 U/ml the sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) to differentiate benign from malignant disease was 81%, 72.4%, 81% and 72.4%, respectively. For a cut off level of 130 U/ml specificity and PPV increased to 96.6% and 96.4%, respectively. Conclusions: The use of CA 19-9 is useful in the diagnosis of patients with suspected bilio-pancreatic malignant disease. The optimization of the normal published value can help to improve accuracy.