Progressive multifocal leukoencephalopathy (PML) is a subacute viral infection of oligodendrocytes by JC virus occurring almost exclusively in immunocompromised patients. By use of partially purified recombinant VP1 as antigen, the IgG response was analyzed by a quantitative ELISA of paired cerebrospinal fluid (CSF) and serum samples. An intrathecal immune response to VP1, defined as an antibody-specificity index of CSF to serum antibody titers > or =1.5, was found in 76% of PML patients (47/62) but in only 3.2% of controls (5/155) (P < .001). Intra-blood-brain barrier synthesis of VP1-specific IgG antibodies is 76% sensitive and 96.8% specific for the diagnosis of PML. Furthermore, the excellent correlation (r = .985) between the plasma cell count in brain tissue and the humoral intrathecal immune response to VP1 in PML patients suggests a role for B cells in this disorder.