Clinical and histological analyses were made of 18 consecutive cases of peripheral artery atherosclerotic aneurysms (PAAA) (common, deep femoral and popliteal arteries) and compared to a group of 10 specimens obtained from the atherosclerotic, non aneurysmal femoral arteries of 10 cadavers with similar characteristics to the 18 patients. Although neither the clinical nor the macroscopic morphological data indicated the presence of an inflammatory process in the PAAA, the histological examination revealed the presence of a considerable infiltrate (greater than 11 inflammatory cells/2116 microns2) in a surprisingly high percentage of cases (4 cases, 22.8%). In 5 other cases (27.7%) the presence of lymphomonoplasmonocytic cells, although less pronounced, was greater than normally seen in atherosclerotic arterial walls (greater than 4 and less than 11 inflammatory cells/2116 microns2). The median number of inflammatory cells present in the media and adventitia varied from 2.2 +/- 0.3 to 13.2 +/- 0.3 per 2116 microns2. These values are significantly different compared to the results of atherosclerotic arteries at the same level (P less than 0.001 Mann-Whitney's U test). These findings of lymphomonoplasmocytic infiltrates in the absence of other pathology, together with an analysis of the data in the literature, makes it possible to presume that the inflammation present is associated with atherosclerosis and is more common in aneurysmal rather than stenotic forms. The pathogenesis may be determined by immune reaction phenomena.