In 21 patients with clinically suspected recurrent rectal cancer showing a mass in the pelvic CT scan, Positron Emission Tomography (PET) was performed to measure metabolic rate of the mass. In 13 of the 21 patients an increased F-18-Fluorodeoxyglucose (FDG) uptake in the suspect tissue could be found. All of these patients were proven to suffer recurrent cancer by biopsy. 8 of the 21 patients showed normal uptake and 7 of them revealed scar tissue by histology. Therefore PET could distinguish clearly between cancer and scar tissue.