A 51-year-old man presented with a 3-month history of intermittent dizziness, headaches, and episodes of right-sided numbness and tingling involving the right hand and foot. Magnetic resonance imaging of the brain with gadolinium revealed a ring-enhancing brainstem lesion with surrounding fluid-attenuated inversion recovery abnormalities. The differential diagnosis included malignant glioma, sarcoidosis, or abscess. The patient began treatment with steroids; the neurological signs and symptoms improved significantly during 3 weeks. Computed tomography of the chest, abdomen, and pelvis showed innumerable small pulmonary nodules and mediastinal lymphadenopathy. Repeated magnetic resonance imaging of the brain, performed 2 weeks after the initiation of steroids, showed an interval decrease in the size of the lesion as well as significant reduction of the surrounding fluid-attenuated inversion recovery signal. Magnetic resonance spectroscopy findings were consistent with a necrotic lesion with significant cell turnover, suggesting either tumor or inflammatory changes. Two weeks after the second magnetic resonance image was obtained, the patient developed progressive neurological signs and symptoms consisting of diplopia, left abducens nerve palsy, cerebellar ataxia, and difficulty ambulating. A third magnetic resonance image showed significant progression of the ring-enhancing mass lesion (Figure, A) as well as massive fluid-attenuated inversion recovery abnormality. A stereotactic pontine biopsy revealed a fungal abscess consistent with blastomycosis, and a fungal culture was positive for Blastomyces dermatitidis (Figure, B and C). The patient's symptoms have improved with systemic antifungal therapy. It is noteworthy that the patient was afebrile, was negative for human immunodeficiency virus, and had no other risk factors for infections.