We report here that the traditional Chinese formulation, Chaihu-jia-Longgu-Muli-tang (CLM), significantly inhibited the increase in intimal thickening in rat carotid artery injured by balloon endothelial denudation, which mimics many aspects of restenosis after percutaneous coronary intervention (PCI) in humans. CLM, Saiko-ka-Ryukotsu-Borei-to in Japanese, is commonly prescribed for symptoms accompanying hypertension and atherosclerosis in Japanese Kampo medical care. CLM administered orally 1 week before and 1, 4 and 8 weeks after balloon injury inhibited the increase in intimal area, intimal/medial ratio and stenosis ratio. To our knowledge, this is the first report demonstrating inhibitory effects of a traditional Chinese formulation on intimal thickening of carotid artery after balloon injury. It is worth noting that CLM maintained its inhibitory effect up to 8 weeks after balloon injury. The reduction in intimal thickening by CLM could have resulted from inhibition of intimal smooth muscle cell proliferation, which was assessed by immuno-histochemical analysis using monoclonal antibody against proliferating cell nuclear antigen. Therefore, CLM may be a favourable candidate for prevention of restenosis after PCI. Moreover CLM may have a therapeutic value in the prevention of atherosclerosis, because restenosis after PCI is considered to be an accelerated atherosclerosis.