A 28-year-old woman complaining of headache, double vision, and drooping of the right upper eyelid for a week was admitted to the hospital. She had given birth 40 days earlier. She had a right blepharoptosis and diplopia, which are findings of right oculomotor nerve paralysis, and abnormal cerebellar function tests on physical examination. Cranial magnetic resonance imaging showed a 1 cm in diameter tumor in the mesencephalon. She was referred for FDG PET-CT study for differential diagnosis of a primary brain tumor and unknown brain metastasis, and to detect the primary tumor if any. FDG PET-CT scan showed decreased FDG uptake in the tumor region and asymmetric, more prominent FDG uptake in the left extraocular muscles. There was no evidence of hypermetabolic focus suggesting a primary malignant tumor outside the brain on a whole-body FDG PET-CT scan and the tumor in the mesencephalon was considered as a primary brain tumor. We report a case of asymmetric FDG uptake in the extraocular muscles resulting from unilateral third cranial nerve paralysis.