Patients with human immunodeficiency virus (HIV) infection have a decreased risk of developing multiple sclerosis (MS) and MS patients very rarely contract HIV infection. We report on a 35-year-old woman with relapsing-remitting MS, who acquired HIV infection 8 years after MS onset. During 7 years of follow-up without combined antiretroviral therapy (cART), CD4+ counts decreased and HIV viremia increased progressively, but slightly. These trends reverted after starting cART, with optimal viro-immunological control. While the patient had many MS relapses before acquiring HIV infection, she had then only one relapse, shortly after HIV infection, despite irregular or no MS therapy. This case contributes to the discussion about MS and HIV potential interactions and describes for the first time the effects of the MS-targeting drug natalizumab in an HIV-positive patient.