An important inflammatory reaction was histologically observed in the wall of an abdominal aortic aneurysm in a 42 year-old patient suffering from multifocal atherosclerotic lesions. Histochemical and ultrastructural studies showed an active-stage wall lymphomonoplasmacellular component. The histological appearance and biochemical and microbiological studies excluded specific aortitis, a rheumatic disease or concomitant infectious processes. The morphological relationships between the atherosclerotic lesion and the infiltrate together with the immunophenotyping features of the infiltrate suggest the possibility of a close correlation between the inflammatory process and the atherosclerotic lesion of the wall.