Background - Coronary irradiation is a new concept to reduce restenosis. We evaluated the feasibility and safety of intracoronary irradiation with a balloon catheter filled with ¹⁸⁸Re, a liquid, high-energy β-emitter. Methods and Results - Irradiation with 15 Gy at 0.5-mm tissue depth was performed in 28 lesions after balloon dilation (n = 9) or stenting (n=19). Lesions included 19 de novo stenoses, 4 occlusions, and 5 restenoses. Irradiation time was 515±199 seconds in 1 to 4 fractions. There were no procedural complications. One patient died of noncardiac causes at day 23. One asymptomatic patient refused 6-month angiography. Quantitative angiography after intervention showed a reference diameter of 2.77±0.35 mm and a minimal lumen diameter of 2.36±0.43 mm. At 6-month follow-up, minimal lumen diameter was 1.45±0.88 mm (late loss index 0.57). Target lesion restenosis rate (>50% in diameter) was low (12%; 3 of 26). In addition, we observed 9 stenoses at the proximal or distal end of the irradiation zone, potentially caused by the short irradiation segment and the decreasing irradiation dose at its borders ('edge' stenoses). The total restenosis rate was 46% and was significantly lower (29% vs 70%, P=0.042) when the length of the irradiated segment was more than twice the lesion length. Conclusions - Coronary irradiation with a ¹⁸⁸Re-filled balloon is technically feasible and safe, requiring only standard percutaneous transluminal coronary angioplasty techniques. The target lesion restenosis rate was low. The observed edge stenoses appear to be avoidable by increasing the length of the irradiated segment.