The use of natalizumab to treat multiple sclerosis (MS) has been associated with the development of progressive multifocal leukoencephalopathy (PML), with 242 PML cases reported as of May 3, 2012. Fortunately, rapid withdrawal of the drug and administration of plasma exchange has allowed survival in many of these patients, but a new problem, immune reconstitution inflammatory syndrome (IRIS), has emerged after drug withdrawal. This report provides an update on PML in natalizumab-treated patients and reviews what is currently known about PML-IRIS in this setting; autopsy findings from a well-studied patient are illustrated. This patient with relapsing-remitting MS had been treated for 4 years with natalizumab, with discontinuation of drug after diagnosis of PML by cerebrospinal fluid polymerase chain reaction testing. Disease was manifested by severe paraparesis and expressive aphasia, which progressed before and after the diagnosis of PML. Immune reconstitution inflammatory syndrome was diagnosed, comfort care was instituted, but demise did not occur until 9 months later. Autopsy showed ongoing severe PML-IRIS, with massive cavitary brain lesions containing abundant perivascular and parenchymal CD8-positive T-cell infiltrates. Bone marrow and spleen, but not brain, contained monoclonal T-cell populations by polymerase chain reaction-based gene rearrangement studies, indicating overstimulation of peripheral T cells; T-cell lymphoma was not identified by morphological or immunohistochemical criteria.