A combined PET/CT scanner has been developed, its clinical application has begun and there is much evidence that it is very helpful, particularly in oncology. In our country, a PET/CT scan, like a dedicated PET scan, is obtained after other conventional imaging modalities, such as CT, are performed. Therefore, patients who receive a PET/CT scan are obliged to have CT scans performed twice, i.e. a conventional CT scan with or without contrast material and a second CT scan as a part of the PET/CT study. Since a radiation exposure by CT can be excessive, the electric current has to be reduced in order to avoid unnecessary radiation exposure by PET/CT scanning, and this results in poor image quality of the CT. A PET study with18F-fluorodeoxyglucose (FDG) is especially useful for staging and re-staging disease in patients with cancer. If patients could undergo a PET/CT scan using the highest quality CT and avoid receiving a conventional CT scan for diagnostic purposes, we would be able to save time for evaluating each patient, save time in ordering and interpreting procedures, and avoid redundant radiation exposure to the patients. All of this should lead to greater costeffectiveness in patient care. If a revised fee of PET/CT with contrast would be 87300 yen, then it could reduce about 7 million yen of medical cost for one thousand tests, as compared with the case in which a CT scan is performed in 80% before a PET scan and another CT scan is recommended after PET in 50%. We have reviewed the clinical usefulness of PET/CT with FDG in oncology, and propose a novel system in applying this device based on evidence of its cost-effectiveness.