Natalizumab (NTM) represents an effective drug for the treatment of relapsing-remitting multiple sclerosis (RRMS). Progressive multifocal leucoencephalopathy (PML) is a potential life-threatening complication of NTM treatment. A close follow-up and MRI monitoring of patients under NTM are required to avoid such devastating complications. The case of a 47-year-old woman with RRMS (EDSS 1.5) treated with NTM for 44 months is reported. The patient had a relapse with mild cerebellar symptomatology and visual complaints. MRI revealed a new area of abnormal signal intensity in the subcortical white matter of the right parietal lobe with mild peripheral enhancement. Visual fields showed scotomata mostly of the left eye. NTM was discontinued. JC virus (JCV) polymerase chain reaction (PCR) in cerebrospinal fluid was negative. The patient received IV corticosteroids for 5 days and then monthly for 2 months with subsequent clinical and MRI improvement. On month 4, she presented with a new relapse with severe ataxia, mild behavioral change, increase of cerebellar symptoms and internuclear opthalmoplegia (EDSS 3.5). MRI showed reappearance of the right parietal lobe lesion, with decreased size and less pronounced contrast enhancement. A new 2-cm lesion was noted in the left cerebellar hemisphere with a speckled pattern of contrast enhancement. JCV PCR was negative and the patient was treated with IV corticosteroids. On month 12, she demonstrated clinical and MRI improvement. Although initially PML was highly suspected in this patient, the clinical and MRI findings were supportive of the presence of immune reconstitution inflammatory syndrome (IRIS).