Multiple Sclerosis (MS) is a chronic inflammatory demyelinating disease of the Central Nervous System (CNS) of supposed autoimmune origin whose etiology still remains unknown. Epidemiological studies suggest that MS pathogenesis could be related to an infection superimposed on a predisposing genetic background. However, at present, no direct evidence for an infectious implication in MS autoimmunity exists. Recently, the potential role of Chlamydia pneumoniae as a causative agent of MS has received increasing attention. After the initial high rates reported for molecular and culture demonstration of C. pneumoniae in cerebrospinal fluid (CSF) of MS patients, the association between C. pneumoniae and MS was intensively investigated with controversial results. Seroepidemiological reports did not show any strong association between C. pneumoniae infection and the risk of MS. Isolation techniques failed to detect C. pneumoniae in CSF and brain tissue of MS patients. Polymerase chain reaction (PCR) evidence of C. pneumoniae in CSF and intrathecal synthesis of anti-C. pneumoniae IgG have been undetectable in MS patients or, if present, were not selectively associated with MS, but were shared by several inflammatory neurological conditions. Nevertheless, in a subgroup of MS patients, an association between PCR positivity for C. pneumoniae in CSF and disease activity was found. Intrathecal production of anti-C. pneumoniae high-affinity IgG predominated in MS progressive forms and metabolically active C. pneumoniae was identified in CSF. C. pneumoniae was recognized in CSF and brain tissue at immunohistochemical, molecular and ultrastructural levels. C. pneumoniae was also able to induce the animal model of MS. This growing body of data does not support a central role for C. pneumoniae as a candidate in MS pathogenesis, but suggests that, in a subset of MS patients, C. pneumoniae could induce a chronic persistent brain infection acting as a cofactor in the development of the disease. Thus, the actual involvement of C. pneumoniae in MS still remains to be elucidated.