Introduction Multiple sclerosis (MS) is an inflammatory demyelinating disease of the central nervous system (CNS). Although MS etiology is unknown, but recent studies have suggested that autoimmune and environmental factors have important roles in its pathogenesis; 1 chronic infections such as Epstein bar virus (EBV), Human herpesvirus-6 and Human herpesvirus-7 have been proposed to have role in MS occurrence. 2-4 The importance of infection, as a risk factor of MS, is supported by abnormal immunological factors found in spinal fluid; although no agent is consistently associated with the disease. 5 Therefore, investigating the role of different chronic infection in MS pathogenesis is reasonable. Mycoplasma pneumoniae (MPn), which is common cause of community-acquired pneumonia, 6 trend to induce numerous CNS manifestations such as encephalitis, aseptic meningitis, polyradiculitis, cerebellar ataxia, and myeliti. 6-8 Neurologic manifestations are the most common nonpulmonary manifestations of MPn infection 9 and up to 7% of patients hospitalized with MPn may have CNS symptoms too. 6 The mechanism of CNS involvement by MPn remains unclear and direct invasion, neurotoxin production, or an immune-mediated mechanism has been proposed. 6 It is showed that in many patients, CSF IgM and IgG markedly exceeded the corresponding serum values 10,11 that may be supported by immune mediated response. Complement-fixing antibodies against MPn are detected in serum and cerebrospinal fluid of patients with MS and the potentially role of MPn in MS pathogenesis is suggested. 11 Acute disseminated encephalomyelitis, optic neuritis, myelitis and neuromyelitis optica 12 are reported to be associated with MPn infection 13-15 so MPn may have a possible role in development of demyelinative disease. This study was done to evaluate MPn seroposivity in patients with MS. Vol. 62, No. 3 (Suppl. 1), March 2012 S-6 Mycoplasma Pneumonia Seroposivity in Iranian Patients with Relapsing-Remitting Multipl Sclerosis: A Randomized Case-Control Study Mohammadali Bahar, 1 Fereshteh Ashtari, 2 Maryam Aghaei, 3 Mojtaba Akbari, 4 Mehri Salari, 5 Shermineh Ghalamkari 6 Immunology Research Center, Tehran University of Medical Sciences, Tehran, 1 Isfahan Neuroscience Research Center, Isfahan University of Medical Sciences, Isfahan, 2,4,5 Islamic Azad University, Tehran, 3 Iinformation Technology Management Research Centre, The University of Isfahan, Isfahan, 6 Iran. Corresponding Author: Fereshteh Ashtari. (f_ashtari@med.mui.ac.ir). Abstract Objectives: Environmental factors, such as different infections, have proposed to be involved in the pathogenesis of multiple sclerosis (MS). This study aimed to the evaluate mycoplasma pneumonia seropositivity, as a common cause of community-acquired pneumonia in patients with relapsing-remitting multiple sclerosis (RRMS). Methodology: Using ELISA method, IgM and IgG antibodies to Mycoplasma pneumoniae were determined in 130 patients with relapsing-remitting multiple sclerosis (85 Remitted and 45 Relapsed) and 50 sex- and age-matched controls. The groups were compared using Kruskal-Wallis test at the significant level of p < 0.05. Results: The median [interquartile range] titer of IgG in remitted multiple sclerosis group was 65.3[51.1-75.2] RU/ml versus 64[52.6-71.4] RU/ml in relapsed group and 57.5[29.2-74.3] RU/ml in control group (p = 0.442). There was not any significant difference between the groups base on median titer of IgM too (p = 0.446). The median [interquartile range] titer of Mycoplasma pneumoniae (MPn) IgG in women was 69.2[56.4-77.4] RU/ml in remitted patients versus 63.85[52.45-71.25] RU/ml in relapsed patients and 55.2[29.17-72.75] RU/ml in controls (p = 0.022). Post hoc analysis demonstrated significant difference between remitted patients and controls (p = 0.002). There was not any significant difference between men in the groups (p = 0.7). Conclusions: Mycoplasma seroposivity in relapsing-remitting multiple sclerosis was not significantly different in various phases of activity of disease compare to controls; but in women, seroposivity of Mycoplasma antibodies were more than controls. Keywords: Relapsing-remitting multiple sclerosis, Mycoplasma pneumonia, ELISA (JPMA 62: S-6; 2012). Original Article