18F-fluorodeoxyglucose positron emission tomography was able to identify previously unknown primary tumors in 2 of 4 patients after an unsuccessful conventional diagnostic workup such as chest radiography, ultrasound, computed tomography, MRI and various endoscopies. The 2 patients in which the primary tumors were detected proved to have a carcinoma of the lung, one of the patients received radiotherapy and chemotherapy after the detection of the primary tumor by FDG PET. The primary tumor of the lung demonstrated no focal FDG uptake after the successive treatment. On the other hand, in one patient with prostatic carcinoma and another in which the primary tumor has yet to be detected, FDG PET was unable to identify the primary tumor. This suggests a limitation of PET studies in detecting cancers. Because of increased glycolysis in cancer cells, FDG PET can be used to detect cancers with its high sensitivity, surveying the entire body non-invasively in one session. PET has the advantage of detecting primary tumors of an unknown origin when compared to conventional diagnostic studies.