We describe a 72-year-old white man with erosive rheumatoid arthritis in whom subacute neurologic and psychiatric symptoms developed after 3 years of treatment with infliximab, prednisone, and methotrexate. White matter demyelination was seen on magnetic resonance imaging of the brain, and progressive multifocal leukoencephalopathy (PML) was ultimately confirmed by brain biopsy. The patient was treated with supportive therapy and discontinuation of disease-modifying antirheumatic drugs, resulting in stabilization of the disease process. The patient survived, but neurologic and cognitive deficits persisted. The distribution and pathology of this patient's disease are unique from almost all reported incidents of oral methotrexate–associated leukoencephalopathy. The pathogenesis of disease may be linked to a T cell–mediated process that is potentially impacted by infliximab. This case provides the first reported evidence that PML can be seen in association with infliximab therapy.