Fluorine-18-fluorodeoxyglucose positron emission tomography ([¹⁸F]FDG PET) was assessed as a method for providing information about the malignancy of orbital tumors. Twelve patients with 13 orbital tumors underwent [¹⁸F]FDG PET followed by biopsy or tumor removal via a transcranial approach. The accumulation ratio between the tumor and the contralateral normal tissue (T/N ratio) was calculated for 10 of the 13 lesions. The T/N ratio in benign lesions was compared with that in malignant tumors. Histological examination identified 7 lesions as malignant: anaplastic astrocytoma of the optic nerve in 1 patient, which recurred as glioblastoma of the optic nerve, malignant lymphoma of mucosa-associated lymphoid tissue type in 1 patient, malignant melanoma in 1 patient, adenoid cystic carcinoma in 2 patients, and adenocarcinoma (unknown origin) in 1 patient. The T/N ratio was 1.06 ± 0.03 (mean ± standard deviation) in benign tumors, and significantly higher at 1.81 ± 0.27 in malignant tumors (p = 0.0027). Both patterns of high and iso uptake of [¹⁸F]FDG were found in orbital pseudotumor. [¹⁸F]FDG PET can determine the malignancy of orbital tumors, but cannot distinguish malignant tumor from inflammatory disease such as pseudotumor.