The authors report on a patient suffering from acute Lyme borreliosis who underwent two consecutive [18F]fluorodeoxyglucose-positron emission tomography (FDG-PET) studies demonstrating the course of the disease. The first FDG-PET study revealed markedly increased glucose metabolism in the brainstem, matching exactly the signal abnormalities exhibited on magnetic resonance images and indicating a brainstem tumor. A second PET scan demonstrated no abnormality in this region, thus reflecting clinical remission following antibiotic therapy. Data in the present case indicate that hypermetabolic findings on FDG-PET studies in the brainstem region should be regarded with caution and that neuroborreliosis must be considered as a possible differential diagnosis.