Sixty patients underwent surgery due to abdominal aortic aneurysms; the group included 30 patients with asymptomatic aneurysm and 30 with ruptured aneurysm. A control group comprised 30 organ donors. Surgical specimens derived from aneurysm or aorta fragments were investigated for Chlamydia pneumoniae DNA using PCR. In asymptomatic aneurysms, DNA was found in 9 cases (29%), and in ruptured aneurysms in 14 cases (49%). In the control group, C. pneumoniae DNA was not detected in an aortic wall. These results suggest that healthy aortic wall is not susceptible to chlamydial infection. A large number of aneurysm infections implies C. pneumoniae role in proteolysis and degradation of the aneurysm wall. The biological effect of this process may cause an enlargement of the aneurysm.