OBJECTIVE: PET / CT is a hybrid technique that produces morphological and metabolic imaging of the whole-body using the radiopharmaceutical 18F-FDG. Because of these possibilities PET / CT has the largest application in clinical oncology for tumor diagnosis and monitoring of diseases and radiotherapy. Using an oral contrast is an integral part of the PET / CT protocol to fit the lumen of the intestine and therefore allowing better visualization of the gastrointestinal tract. The purpose of this study was to evaluate the use of oral contrast in PET / CT diagnosis and answer the following questions: Do oral contrasts enhance or hinder the analysis obtained by PET / CT images; does it affect on accumulation of FDG in the GI tract; does it create artifacts, and whether it is necessary to apply oral contrasts routinely in all patients which are conducting the PET / CT scan or only for those who were instructed to scan for diseases of the GI tract. MATERIALS AND METHODS: 90 patients were divided into 3 groups: 30 patients without oral contrast; 30 patients who used water as an oral contrast; 30 patients who used Gastrografin solution (20 mL Gastrografin in a liter of water). They drank oral contrast through an hour, before the start of the scan. Images were analyzed considering the localization of radionuclide uptake (gastroduodenum, small intestine, colon, rectosigmoid) and considering the appearance of accumulation (without accumulation, segmental, diffuse, focal). RESULTS: Data was collected without technical difficulties. In several respondents nausea had occured due Gastrografin. The majority of the respondents had radionuclide uptake (58,89%) and that majority were patients who used gastrografin and water. There were differences in the type of buildup localization: gastroduodenum often had diffuse accumulation, the small intestine had segmental accumulation. CONCLUSION: Oral contrast agents in PET / CT diagnosis enhance visualization of the intestine, thus allowing easier analysis of fused images. They do not produce clinically significant artifacts, and their impact on the increased accumulation of FDG was also not clinically significant, therefore it can be concluded that it would be useful to apply the oral contrast agents routinely in all patients in PET / CT for diagnosis, not only for patients with gastrointestinal diseases.