Background Multiple sclerosis (MS) is a chronic autoimmune disorder of the central nervous system (CNS) with myelin destructions induced by the immune system (1). The clinical symptoms of MS are highly variable and can include impaired vision and abnormalities in the motor and coordination systems (2). Four general forms for this disease include primary progressive, secondary progressive, relapsing-remitting, and progressive relapsing (3). The disease typically occurs at the age of 18 to 40 and almost in women twice as much as men (4). The incidence of MS in different regions of Iran varies from 5.3 to 74.28 per 100 000 people (5-8). Nevertheless, a study showed that the prevalence of MS was 61 patients per 100 000 people in Hamadan (9). Although the cause of this disease is still unknown, several factors such as genetics, environmental factors, and infectious agents (bacteria and viruses) are known in the etiology of this disease (10). Bacteria as one of the most important infectious agents in human societies can be one of the factors contributing to MS (11). The potential of bacteria in molecular mimicry as an effective mechanism for inducing MS is more than that of viruses (12). Some studies evaluated the role of Acinetobacter, Helicobacter pylori, Spirochetes, Campylobacter, Mycobacteria, and Chlamydia pneumonia in the etiology of MS (13-18). C. pneumoniae is a gramnegative, obligate intracellular, and non-motile bacterium which causes acute respiratory diseases like pneumonia (19). In addition, C. pneumoniae causes other diseases such as atherosclerosis, asthma, encephalitis, and arthritis (20). There are few studies on the association between MS and C. pneumoniae infection. Accordingly, the present study aimed to estimate the serum level of immunoglobulin G (IgG) anti-C. pneumoniae antibodies in MS patients compared with the control subjects. It is hoped that the findings of this study help further understand the etiology of MS.