Circulating levels of tumor necrosis factor α (TNF‐α) are elevated in the patients with abdominal aortic aneurysm (AAA). We investigated TNF‐α expression and cellular infiltration in the walls of AAAs of different sizes. Twenty‐seven surgical specimens of AAAs were categorized according to the maximum aneurysm diameter into a small size group (less than 50 mm in diameter, n = 8; S group), a medium‐sized group (50 to 59 mm in diameter, n = 11; M group), and a large size group (larger than 59 mm in diameter, n = 8; L group). The level of TNF‐α and interleukin‐1β(IL‐1β) in the aneurysm wall was measured by ELISA. Immunohistochemical staining was performed to observe the TNF‐α expression and the infiltration of macrophages and lymphocytes in aneurysm walls. Enzyme‐linked immunosorbent assay showed that the level of TNF‐α in the S group (5.47 ± 3.48 pg/mg protein) was significantly higher (p < 0.05) than that in the M group (2.70 ± 1.33 pg/mg protein) or the L group (1.82 ± 1.21 pg/mg protein). No significant difference in IL‐1β was observed between the S, M, and L groups. Immunohistochemical analysis also showed that TNF‐α was expressed strongly in the S group but was negative or weakly positive in the M and L groups. Furthermore, the expression of TNF‐α was seen mainly where the aneurysm wall showed atheromatous change and macrophage infiltration. These results indicated that the expression of TNF‐α in the aneurysm wall was enhanced in small AAAs, and this enhancement might be related to the infiltration of macrophages.