Today's medical literature shows more and more evidence that Chlamydia pneumoniae plays a role in coronary atherosclerotic disease. This cross-sectional study examines the serostatus for C. pneumoniae of patients with an acute cardiovascular event. A total of 58 patients with acute myocardial infarction or with proven unstable angina (occlusion > or = 70% of at least 1 vessel at coronary angiography) ('patients') were compared with 58 age- and sex-matched patients without a cardiovascular event ('controls'). Serological testing for C. pneumoniae was performed by a microimmunofluorescence test during the first week of the event and 4-6 weeks later. Although more patients from the cardiovascular event group showed a positive history of a respiratory tract infection during 6 weeks preceding hospitalization (18/58 patients, 8/58 controls, p = 0.03), there was no significant difference between the two groups for an acute, chronic or past infection by C. pneumoniae (19/58 patients, 24/58 controls, p = 0.93). In conclusion, contrary to previous published papers, this study could not confirm an association of C. pneumoniae infection with an acute coronary event. Serological testing alone may not be the best way to demonstrate this association. An acute infection by C. pneumoniae, which should have been detected by serological testing, is probably not the origin of the rupture of an atheromatous plaque.