Pathological specimens were collected from 14 unruptured and 13 ruptured aneurysms at the time of clipping and studied in order to assess the underlying mechanism of rupture by investigating degeneration of the aneurysmal wall and possible involvement of apoptosis. Immunohistochemistry with anti-actin antibody showed few smooth muscle cells in the ruptured aneurysms and replacement of the muscularis layer by a fibro-hyalin tissue. However, at least one layer of smooth muscle cells was clearly observed in the unruptured aneurysms. Thus, smooth muscle cells in the wall of the ruptured aneurysms were much more degenerated than those in the wall of unruptured aneurysms. In addition, unruptured aneurysms with an angiographically smooth wall showed well-layered positive staining for anti-smooth muscle actin antibody while those with irregular shapes rarely reacted. We found, for the first time, evidence of DNA fragmentation in the aneurysmal wall. Apoptotic bodies were detected by means of a terminal transferase (TdT)-mediated dUTP biotin nick end labelling technique (TUNEL) and an anti-single-stranded DNA antibody in 54% (7/13) of the ruptured aneurysms. In contrast, apoptotic bodies were found in only 7% (1/14) of the unruptured cases. These results suggest that apoptotic cell death might be involved in the rupture of aneurysms.