BACKGROUND AND OBJECTIVES: Intracoronary stenting now constitute the majority of coronary interventions, and subsequently in-stent restenosis has become a significant clinical problem. Recently, several studies on intracoronary radiation therapy, in patients with in-stent restenosis, have demonstrated a reduction in binary angiographic restenosis and target lesion revascularization compared with control groups. The effects of beta-radiation therapy on non-stented adjacent segments in in-stent restenosis have not been sufficiently evaluated. beta-radiation therapy for in-stent restenosis was performed with a 188Re-MAG3 filled balloon. SUBJECTS AND METHODS: We evaluated, and compared, the effects of beta-radiation therapy on non-stented adjacent segments, in in-stent restenosis, by intravascular ultrasound (IVUS) analysis, between 50 patients who received radiation therapy and 9 controls. The changes (delta: follow-up-post-intervention) of the external elastic membrane (EEM), the lumen and other IVUS variables, were compared between the segments having received radiation therapy and those in the controls. RESULTS: Significant differences were found between the 2 groups, and were as follows: the delta EEM and delta lumen areas were 0.3mm2 and -1.0mm2, (p=0.005) and 0.2mm2 and -1.3mm2, (p<0.001) in those radiated and the controls, respectively. CONCLUSION: In conclusion, compared with the changes on the vessel shrinkage of the control group, there was significant vessel enlargement in the non-stented adjacent segments having received radiation therapy.