The polymerase chain reaction (PCR) was used to identify JC virus (JCV) in the cerebrospinal fluid of two patients with progressive multifocal leukoencephalopathy confirmed by brain biopsy. In addition, JCV viremia was demonstrated by PCR in one case. JCV detection in spinal fluid by PCR may be the first non-invasive technique available for the diagnostic confirmation of progressive multifocal leukoencephalopathy.