Recent evidence has shown the presence ofChlamydia pneumoniaeantigens and nucleic acid in coronary artery atheromas from autopsy patients in South Africa. In this study, the immunocytochemical technique was used to demonstrateC pneumoniaeantigens in atheromas of the aorta in autopsy patients from retrospective aortic atherosclerosis studies at the University of Washington. Hie patients were 34 to 58 years old. Immunoperoxidase staining usingChlamydia-spetiRcmonoclonal antibodies showed one of four fatty streaks and six of 17 fibrous plaques were positive forC pneumoniaeantigens; four control aortic tissues were negative. Two of the positive plaques were from the same patient Double-label immunocy* tochemical staining usingChlamydia-and tissue type-specific monoclonal antibodies demonstrated the antigens in the cytoplasm of macrophages and smooth muscle cells in the atheromatous lesion. This study suggested a wider involvement ofCpneumoniaeorganisms in atherosclerotic lesions of the arterial system than has previously been documented.