OBJECTIVES: to investigate the presence of Chlamydia pneumoniae in the wall of abdominal aortic aneurysms (AAAs) and in the aortas of patients without a history of cardiovascular disease. DESIGN: case-control study. MATERIALS: twenty-six consecutive patients operated for AAA were compared to 17 controls. METHODS: aorta was obtained at surgery or autopsy (controls) and prepared for immunohistochemical (IHC) analysis and culture for C. pneumoniae. Throat swabs from 14/26 patients were analysed by PCR for C. pneumoniae. Blood was obtained from 24/26 patients and from 178 70-year-old males. RESULTS: C. pneumoniae was detected in the aortic aneurysms of 20/26 patients by IHC. C. pneumoniae was cultured from 10 of the 20 IHC-positive patients. Only 1/17 controls was positive for C. pneumoniae by IHC (p=0.0001). PCR was positive for C. pneumoniae in 5/14 patients. Serological analysis by microimmunofluoresence (MIF) showed significantly more high titres of the specific antibodies to C. pneumoniae in patients than in age-matched male controls. CONCLUSIONS: we conclude that C. pneumoniae is often present in AAAs in a viable form and that C. pneumoniae is linked to the pathogenesis of AAA. Copyright 2000 Harcourt Publishers Ltd.