Background: The purpose of this study was to assess the predictive effect of preoperative CEA and CA 19-9 levels on the prognosis of colorectal and gastric cancer patients. Materials and Methods: CEA and CA 19-9 were evaluated preoperatively in patients undergoing surgery for colorectal cancer (n=116) and gastric cancer (n=49). Patients with CEA levels <5 ng/mL were classified as CEA Group 1, 5-30 ng/mL as CEA Group 2 and >30 ng/mL were classified as CEA Group 3. Similarly the patients with a CA 19-9 level <35 U/mL were classified as CA 19-9 Group 1, with 35-100 U/mL as Group 2 and with >100 U/mL as Group and 3. TNM stages and histologic grades were noted according to histopathological reports. Patients with a TNM grade 0 or 1 were classified as Group A, TNM grade 2 patients constituted Group B and TNM grade 3 and 4 patients constituted Group C. Demographic characteristics, tumor locations and blood types of the patients were all recorded and these data were compared with the preoperative CEA and CA19-9 values. Results: A significant correlation between CA 19-9 levels (>100 U/mL) and TNM stage (in advanced stages) was determined. We also determined a significant correlation between TNM stages and positive vlaues for both CEA and CA 19-9 in colorectal and gastric cancer patients. In comparison between CEA and CA 19-9 levels and age, gender, tumor location, ABO blood group, and tumor histologic grade, no significant correlation was found. Conclusions: Positive levels of both CEA and CA 19-9 can be considered to indicate an advanced stage in colorectal and gastric cancer patients.