Abstract: Evaluation of single and combined testing of carcinoembryonic antigen (CEA), carbohydrate antigen 19-9 (CA 19-9), carbohydrate antigen (CA 242) and alphafetoprotein (AFP) in serum for diagnosis of gastrointestinal tumors were aimed in the present study. Sera of 28 healthy individuals and 181 patients with gastrointestinal tumors including colorectal cancer (n=50), pancreatic cancer (n=27), hepatocellular carcinoma (n=86), gastric cancer (n=18) were evaluated for different tumor markers. Complete liver functions were also determined. In colorectal cancer patients, single determination of CEA or CA 242 showed the highest sensitivity (48%) and combined determination improved the sensitivity to 72%. In pancreatic cancer patients, the combination of CA 19-9 with CA 242 did not increase the sensitivity above that (70.4%) of CA 19-9 alone. In HCC, AFP was the highly sensitive tumor marker. In addition, a significant positive correlation (r=0.816) was found between AFP and ALT in HCC patients. All evaluated tumor markers showed lower sensitivities (< 22%) for gastric cancer. Combined detection of CEA and CA 242 improved the sensitivity in colon cancer. CA 19-9 showed the best sensitivity for pancreatic cancer. AFP was the most sensitive tumour marker for HCC. None of the evaluated tumour markers had sufficient sensitivity to be taken as a marker for gastric carcinoma.