Chlamydia pneumoniae is a common cause of acute respiratory infection and has been hypothesized to cause several chronic diseases, including lung cancer. The purpose of this article is to identify, describe, and critically examine the published studies on the association between C. pneumoniae infection and risk of lung cancer. In the six studies identified, previous C. pneumoniae infection was defined on the basis of serologic criteria, which varied between studies. All studies reported elevated relative risk estimates for the association of serologic evidence of infection and risk of lung cancer. The three studies in which past infection was defined based on testing of prediagnostic blood specimens tended to have weaker results (odds ratio range, 1.2-2.1) than those based on postdiagnostic blood specimens (odds ratio range, 1.4-9.9). Selection bias, measurement error, and inadequate control for confounding are concerns in some of these studies. Nevertheless, results were relatively consistent, supporting a causal association. Inflammation caused by chronic infection with C. pneumoniae may be involved in the carcinogenic process but this relationship will be difficult to further define through serologic data. To better understand the nature of this association, both experimental study designs, such as those based on animal models or randomized controlled antibiotic treatment trials in humans, and observational study designs (e.g., studies that involve detection of C. pneumoniae in pulmonary specimens obtained before cancer onset) could be explored and may shed additional light on this important association.