Background - The 'edge effect,' late lumen loss at the margins of the treated segment, has become an important issue in the field of coronary brachytherapy. The aim of the present study was to assess the edge effect in noninjured margins adjacent to the irradiated segments after catheter-based intracoronary β-irradiation. Methods and Results - Fifty-three vessels were assessed by means of 3-dimensional intravascular ultrasound after the procedure and at 6- to 8-month follow-up. Fourteen vessels (placebo group) did not receive radiation (sham source), whereas 39 vessels were irradiated. In the irradiated group, 48 edges (5 mm in length) were identified as noninjured, whereas 18 noninjured edges were selected in the placebo group. We compared the volumetric intravascular ultrasound measurements of the noninjured edges of the irradiated vessels with the fully irradiated nonstented segments (IRS, n=27) (26-mm segments received the prescribed 100% isodose) and the noninjured edges of the vessels of the piacebo patients. The lumen decreased (6 mm3) in the noninjured edges of the irradiated vessels at follow-up (P=0.001). We observed a similar increase in plaque volume in all segments: noninjured edges of the irradiated group (19.6%), noninjured edges of the placebo group (21.5%), and IRS (21.0%). The total vessel volume increased in the IRS in the 3 groups. No edge segment was subject to repeat revascularization. Conclusions - The edge effect occurs in the noninjured margins of radiation source train in both irradiated and placebo patients. Thus, low-dose radiation may not play an important role in this phenomenon, whereas nonmeasurable device injury may be considered a plausible alternative explanation.