Chlamydia pneumoniae is a major cause of acute respiratory tract diseases in human and has been responsible for both endemic and epidemic pneumonia. In addition, this organism is thought to play some roles in other clinical manifestations, like coronary artery diseases, asthma and sarcoidosis. This is the first Saudi Arabian study that investigates the prevalence of this organism in Saudi Arabia, its pathogenesis and immunological response. The study employed a wide range of advanced techniques and methods, including culturing , detection by immunoflourescence, polymerase chain reaction (PCR), electron microscopy, antichlamydial activity assays and the most recent advanced technique of microarray technology.