This study examined the effects of endovascular irradiation on cell proliferation and apoptosis after overstretch balloon angioplasty in a normolipemic swine model of restenosi. Twenty five swine underwent oversized balloon angioplasty in the LAD and/or LCX,and in 13 of them the procedure was followed immediately by endovascular radiation using a guide wire with 3 cm long 192 Ir line source to deliver 20Gy to overdilated segments. All the animals were killed at 3 and 30 days. Cell proliferation was estimated by immunostaining for proliferation cell nuclear antigen (PCNA). Apoptosis was identified by the terminal uridine nick end labeling(TUNEL). The results showed that endovascular radiation significantly reduced the pcna positive cells compared with controls 3 days and 30 days after initial injury. There were no quantitative differences in the amount of TUNEL labeling cells among irradiated and control vessels in any region examined 3 days and 30 days after balloon injury. Endovascular radiation may reduce restenosis by inhibiting the cell proliferation, not by enhancing cell apoptosis.