In a case-control study that included a total of 98 patients and 83 controls, the possible link between various pathogens and abdominal aortic aneurysms was investigated. For 68 patients with abdominal aortic aneurysm and age-matched controls, no differences were detected in the levels of immunoglobulin (Ig)A and IgG Chlamydiaceae and Chlamydophila pneumoniae antibodies. Patients with IgA titers positive for Chlamydophila pneumoniae showed progressive disease (defined as an annual increase of the aneurysm diameter of > or = 0.5 cm) more frequently than patients with negative IgA titers (p = 0.046). Polymerase chain reactions performed to detect DNA for Chlamydophila pneumoniae, Chlamydia trachomatis, Chlamydophila psittaci, human cytomegalovirus, Borrelia burgdorferi and Helicobacter pylori in tissue specimens of 30 patients and 15 controls were negative. In summary, Chlamydophila pneumoniae may contribute to aortic aneurysm disease progression, but DNA of this and other pathogens was not found in patients' specimens.