Psychotic disorders in patients with multiple sclerosis (MS), although reported in the literature, are quite rare. The maniac psychosis is increased in MS patients, especially after steroid use, but a pure paranoid (delusional) state is very uncommon. We report a case of a patient with MS complicated by Capgras' syndrome. This disorder, characterized by misidentification and also known as "illusion of double", was first described by the French psychiatrist Joseph Capgras in 1923. Our patient was a 36-year-old female, with a negative psychiatric history; the diagnosis of MS dated back to the age of 18. Subsequently, after a treatment with high dosage of steroids for optic neuritis, her psychiatric symptoms (delusion of references) began and she was then treated with clozapine. Thereafter she had repeated relapses. Immunomodulatory treatments with beta-interferon first and azathioprine then were stopped for intolerance. She came to our hospital for a new relapse with severe dynamic ataxia. After a treatment with corticosteroids the patient developed a paranoid disorder characterized by persecutory delusion (illusion of double) towards her husband. Treatment with glatiramer acetate and quetiapine improved her neuropsychiatric condition.