A 22-year-old Haitian man had a 15-month course of progressive meningitis accompanied by multiple cerebral infarcts. Multiple areas of stenosis and occlusion in all branches of the circle of Willis, and hypertrophy of collateral perforating vessels at the base of the brain in a "puff of smoke" appearance typical of moyamoya disease were seen on cerebral angiogram 5 months before the patient died. At autopsy, the patient had meningovascular syphilis and a necrotizing encephalitis with massive treponemal invasion of the brain, the pathology of late-stage degenerative, "quaternary", neurosyphilis. The patient was also infected with human immunodeficiency virus (HIV). Retrovirus-like particles 100 nm in diameter with dense cores were seen by electron microscopy. Nucleic acid obtained from the patient's brain contained sequences homologous to HIV DNA as determined by dot blot hybridization. The moyamoya-like radiologic appearance of neurosyphilis has not been previously described. The autopsy finding of quaternary neurosyphilis in a patient with HIV infection supports the hypothesis that retrovirus may alter the natural history of syphilitic infection.