A 74-year-old man with a 9-month history of treatment for metastatic small cell lung carcinoma presented with 3 months of gradual-onset small steps and stooped posture. His medical history was unremarkable except for pneumothorax in his 30s. Neurologic examination revealed bradykinesia, rigidity in the right extremities, stooped posture, and hyperreflexia in all extremities. Brain MRI showed a ring-enhanced lesion in the midbrain involving the left substantia nigra (figure), which was not evident on MRI 4 months earlier. Although he underwent whole-brain irradiation with a marked decrease in the lesion size, his parkinsonism did not remit. He was unresponsive to levodopa up to 300 mg/d. His parkinsonism did not worsen, and he died 4 months after diagnosis.