We report a patient with meningovascular syphilis who had a dorsal midbrain syndrome, cognitive dysfunction, and a left peripheral seventh nerve palsy. Magnetic resonance imaging (MRI) disclosed a large lesion of the midbrain and thalamus with intense enhancement of the interpeduncular cistern, both of which resolved after treatment with intravenous penicillin. The clinical features, radiographic appearance, and response to therapy suggest that this lesion was a focal syphilitic inflammatory process, or gumma. We conclude that MRI with intravenous contrast may reveal the full spectrum of pathologic involvement in neurosyphilis and, in certain situations, may obviate the need for biopsy of an associated mass lesion. © Williams and Wilkins 1991.