Fifty-one consecutively removed abdominal aortic atherosclerotic aneurysms were graded as mild, moderate, or severe, according to the amount of chronic inflammation and fibrosis present. Mild chronic inflammation and fibrosis were present in 72.5% of the aneurysms and moderate changes in 15.7%. The presence of the latter group indicated that there was no sharp distinction between the usual atherosclerotic aneurysm with mild chronic inflammation and those with severe changes (inflammatory aneurysms). Patients with the latter comprised about 12% of the total and showed no significant clinical difference from those with ordinary atherosclerotic aneurysms, apart from difficult operative dissection that led to a raised operative mortality. The clinical and pathological findings suggest that inflammatory aneurysms of the abdominal aorta are simply atherosclerotic aneurysms that show an unusual accentuation of the chronic inflammation and fibrosis that may be observed in relation to atherosclerotic aneurysms.