A 74-year- man was hospitalized for subacute aphasia and right hemiparesis. He had had chronic lymphoid leukemia for 11 years and had been treated 5 months earlier with 3 courses of fludarabine. Magnetic resonance imaging showed lesions of the temporo-occipital white matter compatible with progressive multifocal leukoencephalitis (PML). The presence of JC virus was demonstrated first by in situ hybridation after a brain biopsy and second with polymerase chain reaction on CSF. The diagnosis of PML was confirmed. The causality of fludarabine treatment is discussed.