Seroepidemiological studies have shown an association between Chlamydia pneumoniae and atherosclerosis, the risk of acute myocardial infarction and abdominal aortic aneurysms (AAA). Several studies have detected C. pneumoniae in atherosclerotic lesions from coronary and carotid arteries, in AAA, and in sclerotic aortic valves. However, culturing of C. pneumoniae is difficult and has seldomly succeeded from atherosclerotic lesions. Thus, the pathogenicity is unknown, and the significance of detecting the organism is unresolved. Nevertheless, in a large observational study comparing the risk of cardiovascular events among recipients of macrolide versus pencillins, macrolide treatment reduced the risk of such events after relevant adjustment. Furthermore, in two out of three minor randomized clinical trials were patients with ischaemic heart disease were randomized into antibiotic treated and placebo groups, a significant reduction in serious end-points were noticed in patients receiving macrolide. Similarly, two other minor randomized trials showed that macrolide treatment inhibited growth of small AAA. Macrolide therapy thus seems potential to improve the outcome of severe ischaemic heart disease, and growth of AAA. If true, it not known whether this is transient because of macrolides non-specific anti-inflammatory effect or latent infection, or permanent because of eradicating C. pneumoniae organisms. In order to clarify this, large and long term randomized trials are needed, as well as diagnostic methods that can differentiate between individuals who are or are not infected with C. pneumoniae. The latter are needed in order to clarify the impact of the presence of C. pneumoniae and to avoid overconsumption of antimicrobials, which can result in serious ecological problems.