Purpose: The aim of this study was to evaluate the usefulness of whole body positron emission tomography (PET) using 18F-fluorodeoxyglucose (18F-FDG) for cancer screening in asymptomatic subjects. Materials and Methods: The subjects were 1,762 men and 259 women who voluntarily underwent 18F-FDG PET for cancer screening as a part of a routine health examination. Final diagnosis was decided by other diagnostic studies, pathological results or clinical follow-up for 1 year. Results: Of 2,021 subjects, 40 (2.0%) were finally proved to have cancer. Abnormal focal 18F-FDG uptake suggesting malignancy was found in 102 subjects (5.0%). Among them, 21 subjects (1.0%) were proved to have cancer. Other tests in the routine health examination could not find 9 of 21 cancers (42.9%) detected by PET. The sensitivity, specificity, positive predictive value, and negative predictive value of PET for cancer screening were 52.5%, 95.9%, 20.6%, and 99.0%, respectively. Pathologies of cancers missed on PET were adenocarcinoma (n=9; 3 colon cancers, 3 prostate cancers, 2 stomach cancers, and 1 rectal cancer), differentiated thyroid carcinoma (n=6), bronchioalveolar cell carcinoma (n=2), urinary bladder cancer (n=1), and melanoma (n=1). More than half of cancers which were not detected by PET were smaller than 1 cm in diameter. Conclusion: 18F-FDG PET might be useful for cancer screening in asymptomatic subjects due to its high specificity and negative predictive value and play a supplementary role to the conventional health check-up, but it could not replace due to limited sensitivity for urological cancers, small-sized tumors and some hypometaboic cancers. (Nucl Med Mol Imaging 2008;42(6):444-450)