Progressive multifocal leukoencephalopathy (PML) is a demyelinating condition secondary to lytic infection of oligodendrocytes by JC papovavirus occurring in the context of immunodeficiency. Its aggravation after initiation of antiretroviral therapy is part of the immune restoration syndrome (IRS), often fatal if not adequately managed. We report a clinical observation highlighting the dual role of MRI on the one hand in the diagnosis of PML; allowing the discovery of an HIV infection in our case; and other parts in the follow-up of this pathology which can be complicated by a life-threatening IRS. Cerebral MRI is the main imaging medium for the presumptive diagnosis of PML. In MRI, MRI can be used to assess worsening of lesions by increasing their size as well as by the presence of a contrast enhancement after gadolinium injection indicating inflammation. Thus the use of cerebral MRI in a fragile field such as HIV should be performed periodically even in the absence of suggestive clinical signs, to ensure early diagnosis and adequate management and therefore a better prognosis