Three patients, in whom the diagnosis of Borrelia burgdorferi infection was unknown for several years, developed a biphasic involvement of the central nervous system: an acute brain-stem dysfunction was followed up, in two patients, by a progressive, disabling myelitis and, in one patient, by further relapsing-remitting episodes of severe multifocal rhombencephalitis. The most consistent cerebrospinal fluid abnormalities in the analysis of sequential specimens were elevated total IgM levels that normalized after penicillin therapy. The neuropathologic findings in one patient showed microgliosis and meningovascular involvement of the central nervous system, resulting in two ischemic infarcts in the myelencephalon. Few spirochetes were localized in the leptomeninges and around subependymal vessels of the fourth ventricle. The vascular element consisted of an obliterative inflammatory vasculopathy in the medullary parenchyma. This study (1) provides pathologic evidence that a vascular disease induced by B burgdorferi is a pathogenetic mechanism for cerebrovascular diseases, and (2) emphasizes the similarities between neuroborreliosis and neurosyphilis.