Whole-body FDG-PET has proved effective in the diagnosis of recurrent colorectal cancer and in the monitoring of cancer therapy.In this study, we analysed how FDG-PET affects the management of patients with colorectal cancer in the preoperative period. We aimed to evaluate the potential role of FDG-PET in the diagnosis of primary cancer, detection of locoregionel lymph node and distant metastases. A prospective study of 26 patients, with histologically diagnosed or clinically high-suspected colorectal cancer, was undertaken. Patients were evaluated by conventionel imaging methods (chest X-ray, multislice computed tomography and colonoscopy) and FDG-PET. The results of these investigations were compared independently.FDG-PET detected 28 primary colorectal tumors in 26 patients. (3 patients had syncronous tumors) while CT detected only 19. FDG-PET had a sensitivity of 96% and positive predictive value of 100% for the diagnosis of primary cancer. On the other hand, CT had a sensitivity of 68% and positive predictive value of 95% (p<0,05). There was no difference between FDG-PET and CT in detecting the locoregionel lymph nodes and hepatic metastases. There was a statistically significant difference in the evaluation of extrahepatic distant metastases. FDG-PET detected 10 unexpected extrahepatic distant metastases in 2 patients and had a sensitivity of 89% for detecting additional metastatic disease, whereas CT had 33%. FDG-PET resulted in altered management for 8 patients of whom 3 avoided inappropriate surgery. Of this 8 patients; 1 was underwent paliative resection because of unexpected metastases and 2 were underwent extended resection because of syncronous tumors. It is concluded that whole-body FDG-PET is highly sensitive for the diagnosis and staging of patients with colorectal cancer. Especially FDG-PET is superior to CT for the detection of distant metastases at all sites except the liver. Its use in conjunction with CT result in a more accurate selection of patients for surgical treatment with curative intent.