Chlamydia pneumoniae has recently been implicated in the pathogenesis of several neurological diseases. As an intracellular parasite with its unusual life cycle it is able to circumvent the immune system and to persist in the organism. It has the ability to modify the function of the infected cell and supposedly induce autoimmune reactions. These properties can make it pathogenic in several chronic neurological diseases including multiple sclerosis, atherosclerosis, stroke, Alzheimer dementia and giant cell arteritis. The evaluation of the available, often contradictory, data that are based on various different methods is not easy. The importance of the issue is enhanced by the potential need for antibiotic treatment.