Progressive multifocal leukoencephalopathy (PML) is a demyelinating disease of the brain caused by the JC virus. It occurs in association with immunodepression due, for instance, to a hematological malignancy, HIV infection, or immunosuppressive therapy for an organ transplant or systemic disease. We describe the fourth reported case of PML in a patient receiving immunosuppressants for Wegener's granulomatosis. A 71-year-old woman receiving azathioprine and glucocorticoid therapy experienced onset of right-sided hemiplegia within a few days, became comatose, and died within a few days. MRI of the brain showed a subcortical lesion in the left parietal lobe generating low signal on T1 images and high signal on T2 images. The initial diagnosis was cerebral vasculitis. However, the postmortem examination showed PML. The diagnosis of PML rests on JC virus detection in the cerebrospinal fluid by PCR assay and on demonstration in a brain biopsy of the typical histological pattern with presence of the JC virus within the demyelinated lesions. No specific or effective treatments are available. Immunosuppressant drugs should be discontinued if possible.