Introduction: Routine surveillance of colorectal cancer includes serial measurements of CEA levels. Although not routinely indicated Ca 19-9 is also a tool for recurrence. When any of these serum markers is elevated during follow up, this could represent a recurrence. The management of elevated tumor marker levels include clinical exams, endoscopy and conventional imaging–ultrasound, CT, MRI. Objective: To evaluate the positive predictive value of CEA and Ca19-9 as tumor markers for recurrent colorectal cancer in cases where conventional imaging and endoscopic studies fail to localize disease. Materials and methods: A total of 75 patients with elevated CEA and/or Ca19-9 serum levels and negative endoscopic exam as well as negative abdominal CT and Chest X-ray were included in the study. CEA levels were tested in 50 patients. Ca 19-9 was tested in 65 patients. 34 of the patients had both markers tested. All patients underwent whole body 18F-FDG PET/CT. Patients with negative of equivocal PET scan were further followed up (10 to 24 months). Results: Based on the reference standard – the results from PET/CT, if positive and the results from follow-up in cases of negative or equivocal scans, the positive predictive value of Ca 19-9 was 84% and that of CEA -83%. There was no significant difference in the PPV of Ca19-9 and CEA. Conclusion: Elevated CEA and Ca 19-9 levels in patients under active surveillance after operation for colorectal cancer have high positive predictive value for recurrence, even in cases where conventional work-up – endoscopy and CT don’t localize disease., Yana Bocheva, Pavel Bochev, and Literatura