To evaluate the effect and mechanism of hypercholesterolemia on the sequence of changes involved in apoptosis, which counteract proliferation due to the restenosis process after vascular injury. Iliac arteries of hypercholesterolemic (HC) and normocholesterolemic (NC) rabbits were examined 1, 3, 7 & 14 days after balloon injury (N = 5 to 7 animals on each occasion) using TUNEL, immunohistochemical staining of PCNA, macrophages, smooth muscle actin and p53. In media immediately after injury, apoptosis occurred comprehensively and then decreased. HC did not affect this early post-injury apoptosis but significantly (p<0.01) increased apoptosis 14 days later (D14). Immediate apoptosis in media was followed by active proliferation. HC sustained a high activity of proliferation until D14 (p<0.01). The changes of immunoreactivity caused by p53 over the same 14 day period parallels that of apoptosis. In intima, where cells were scarce initially, proliferative activity reached a peak at D7 and then decreased. HC significantly (p<0.05) enhanced proliferation at D14. In intima proliferation was accompanied by a later lower level apoptosis. HC significantly (p<0.05) enhanced this lower level stage of apoptosis at D14. These effects of HC on apoptosis and proliferation result in significantly increased areas of intima and media (p<0.01). The fundamental difference between HC & NC was the infiltration of macrophages in HC, which were colocalized with apoptosis and proliferation activities. In conclusion, balloon injury induces early massive p53-associated apoptosis followed by proliferation in media, whereas in intima, it induces active proliferation followed by a lower activity apoptosis. Hypercholesterolemia does not affect the early post-injury apoptosis but enhances proliferation and lower level apoptosis at a later stage, which in turn results in intimal and medial hyperplasia.