Abstract Cellular infiltrates consisting mainly of lymphocytes are commonly present in the arterial wall in abdominal aortic aneurysm (AAA). However, despite this, the precise nature of these populations has to date not been described in detail. The aim of this study was to perform an exhaustive phenotypic characterisation of the infiltrating lymphocytes in order to define the inflammatory process that develops in AAA. For this purpose, AAA-infiltrating lymphocytes from 25 fresh aneurysm wall samples and, as a control, peripheral blood (PB) lymphocytes from the same patients were purified. The expression of lineage specific markers, maturation molecules, activation antigens and adhesion molecules on T and B lymphocytes was examined by triple-colour immunofluorescence and flow cytometry analysis. The phenotype of AAA-infiltrating lymphocytes was compared with that of PB of the patients with AAA. The results reveal that AAA-infiltrating B and T lymphocytes consist of activated memory cells, with specific homing properties. In addition, they express molecules of B–T co-stimulation and, occasionally, exhibit phenotypical features indicative of the ectopic formation of lymphoid structures. These findings are discussed in the light of the similarities shared with the lymphoid infiltration occurring in other chronic autoimmune/inflammatory disorders.