Purpose: It is important to differentiate malignant from benign lesions of intraocular masses in choosing therapeutic plan. Biopsy of intraocular tumor is not recommended due to the risk of visual damage. We evaluated the usefulness of F-18-FDG PET imaging in diagnosing intraocular neoplasms. Materials and Methods: F-l8-FDG PET scan was performed in 13 patients (15 lesions) suspected to have malignant intraocular tumors. There were 3 benign lesions (retinal detachment, choroidal effusion and hemorrhage) and 10 patients with 12 malignant lesions (3 melanomas, 7 retinoblastomas and 2 metastatic cancers). Regional eye images (256*256 and 128*128 matrices) were obtained with or without attenuation correction. Whole body scan was also performed in eight patients (3 benign and 6 malignant lesions). Results: All malignant lesions were visualized while all benign lesions were not visualized. The mean peak standardized uptake value (SUV) of malignant lesions was 2.64+-0.57 g/ml. There was no correlations between peak SUV and tumor volume. Two large malignant lesions (>1000 mm3 ) showed hot uptake on whole body scan. But two medium-sized lesions (100-l000 mm3) looked faint and two small (<100 mm3) lesions were not visualized. The images reconstructed with 256*256 matrix showed lesions more clearly than those with 128X128 matrix. Conclusion: F-18-FDG PET scan is highly sensitivity in detecting malignant intraocular tumor. For the evaluation of small-sized intraocular lesions, whole body scan is not appropriate because of low sensitivity. A regional scan with sufficient acquisition time is recommended for that purpose. Image reconstruction in matrix size of 256*256 produced clearer images than the ones in 128X128, but it does not affect the diagnostic sensitivity. (Korean J Nucl Med 1999;33:439-451)