Background: The use of disease modifying treatments (DMTs) in multiple sclerosis (MS) has been well studied and has shown to reduce clinical relapses, MRI activity and to some extent disability progression. Dimethyl fumarate is a commonly prescribed oral MS DMT that has proved to be efficacious in MS. Flushing and gastrointestinal symptoms are common side effects, but rare opportunistic infections can occur especially in the setting of lymphopenia. Objectives: To present a case of systemic Listeria monocytogenes in a patient on dimethyl fumarate Methods: Not applicable Results: Case Presentation: A 68-year-old female with long-standing MS (diagnosed in 2003) presented to the hospital with two days of intermittent fevers (maximum 101?), malaise, fatigue, diffuse abdominal pain, nausea, body aches and insomnia. At time of presentation, she was clinically and radiologically stable on dimethyl fumarate for 5 years. Prior DMTs included glatiramer acetate, interferon beta, and natalizumab. Her last absolute lymphocyte count (ALC) prior to admission was 0.79 k/cu mm which was obtained several months prior to hospitalization. Her ALC prior to start of Tecfidera was 1.9 k/cu mm and a month after start was 0.93 k/cu mm. Over the next 5 years her ALC ranged between 0.6-0.96 k/cu mm. During hospitalization, multiple diagnostic studies were obtained including blood cultures, lumbar puncture and MRI. Her ALC on admission was 0.46 k/cu mm and her nadir ALC during hospitalization was 0.22 k/cu mm. Blood cultures revealed Listeria monocytogenes bacteremia. Spinal fluid analyses were unremarkable. Her brain MRI did not reveal any signs of new MS disease activity or central nervous system infection. Exposures for listeria may have been bad lettuce, frozen vegetables or raw steak. During the time of her infection she remained clinically stable with regards to her MS. As of last visit, she remains clinically stable on Tecfidera with an ALC of 0.89 k/cu mm and no new or recurrent infections. Conclusions: Dimethyl fumarate is an efficacious DMT that modulates the immune system and in some cases may result in immunosuppression. Common side effects of dimethyl fumarate are well described, but ongoing vigilance is required to identify less common, potentially life-threating side effects. Systemic symptoms of unknown origin in a patient on dimethyl fumarate need to be extensively worked up, including obtaining blood cultures and spinal fluid analyses to rule out rare opportunistic infections.