Abstract： Objective To determine the mid-term effects of cutting balloon angioplasty (CBA) on in-s tent restenosis. Methods A total of 69 patients with in-stent restenosis were divided into 2 groups rand omly: cutting balloon angioplasty and plain old balloon angioplasty. The mechan isms of restenosis and dialation results were determined by quantitative coronar y angiography and intravascular ultrasound. Follow-up was performed.Results The procedural success rate was 100% without death and acute closure. One patie nt experienced dissection at the distal end of the stent and needed another sten t. The mean follow-up period was 6.7±2.3 months. The final re-restenosis rate was 15% and 18% at 3 months and 6 months respectively, markedly lower than after plain old balloon angioplasty (38% and 43%). Acute gain by intravascular ultrasound (IVUS) was 1.72±0.52mm after cutting balloon angioplasty, higher than 1.15±0.54mm after plain old balloon angioplasty. The lumen diameter late loss in the cutting balloon group was 0.26±0.05mm and 0.38±0.06mm at 3 months and 6 months respectively, significantly lower than for those in co nventional balloon group (0.78±0.19mm and 0.89±0.16mm, respectivelly, P<0.001). As shown by IVUS, the main mechanism of cutting balloon angiopla sty was marked reduction of plaque area without significant increase of vessel a rea (less vessel trauma).Conclusion Cutting balloon angioplasty is feasible and effective for the treatment of in-s tent restenosis with less vessel trauma.