A flood of data indicate the importance and the relevance in both respiratory and extrapulmonary disease of Chlamydia pneumoniae infection today. Antibody prevalence rates in Western countries reach 50% in the adult population and remain high in old age suggesting a high rate of reinfection. Chlamydia pneumoniae can cause upper respiratory tract infections, like pharyngitis and sinusitis as well as bronchitis and more than 10% of community-acquired pneumonia. Recent data suggest a possible association of Chlamydia pneumoniae infection with wheezing and adult-onset asthma. However, the most intriguing hypothesis pertains the possible association between Chlamydia pneumoniae infection and atherosclerosis.