Tumor markers have been used for detecting and monitoring various malignant diseases. However, as the sensitivity and specificity of the tumor markers are not enough for detecting malignant diseases, clinical usefulness of them are limited. The aim of this study was to determine the rate of false positivity of some tumor markers in acute pancreatitis. For this aim, carbohydrate antigen 19-9 (CA 19-9) and carcinoembryonic antigen (CEA) levels were measured in acute pancreatitis. All 61 patients (37 women, 24 men) with acute pancreatitis and a total of 74 patients (39 women, 35 men) with peptic ulcus and gastritis were enrolled in to the study. All of our patients had mild acute pancreatitis, according to the Atlanta criteria. Tumor markers of patients with acute pancreatitis were examined in the first 24 hours. Serum levels of CA 19-9 in acute pancreatitis were significantly higher than the control group. In patients with acute pancreatitis, 36% of CA 19-9 levels were above the upper limit. There was no difference in CEA levels between the study groups. CA 19- 9 levels could be elevated in acute pancreatitis. For this reason tumor markers should not be used for screening but they should be used for only specific indications. [Turk J Cancer 2007;37(1):11-15]