We describe the clinical and therapeutic course of a 52-year-old patient with multiple sclerosis in whom progressive multifocal leukoencephalopathy (PML) developed after 12 months of therapy with natalizumab. The patient was hospitalized 2 months after the onset of neurologic and psychiatric symptoms and was treated with plasma exchange and immunoadsorption to eliminate natalizumab. After a brief improvement, he became critically ill with an apparent episode of immune reconstitution inflammatory syndrome. Steroid-pulse therapy led to stabilization of the patient's condition and clinically significant recovery. This case illustrates that prompt diagnosis and treatment may improve the outcome in patients with severe PML associated with natalizumab therapy.