The aim of this study was to evaluate the incidence and the cause of "edge restenosis" after implantation of high activity 41.1 microCi +/- 1.2 microCi = 1520 kBq +/- 44 kBq, beta-emitting (55Co) stents. Proton bombarding in cyclotron has brought the radioactivity. Intravascular ultrasound (IVUS) investigation has been completed in 10 patients. The angiographies performed at 6 month revealed restenosis >50% in 5 cases (50%). The analysis of edges (5 mm distally and proximally to the last stent struts) showed no significant changes in TVV (187.3 +/- 62.60 mm3 and 176.9 +/- 53.5 mm3) but PMV increase significantly (i.e. neointimal proliferation) from 61.9 +/- 31.2 mm3 to 82.2 +/- 43.4 mm3 (p<0.04) and was the major contributor (from 66%) to lumen volume loss (125.4 +/- 40.7 mm3 and 94.7 +/- 22.2 mm3, p<0.02). In conclusion, neither statistically significant positive nor negative remodelling at the "stent edges", were present. Statistically significant increase in plaque +/- media volume (i.e. neointimal hyperplasia) and reduction in lumen volume were found. The cause of "edge restenosis" was especially (from 66%) due to increase in plaque +/- media volume (i.e. neointimal hyperplasia). Probably, main reason for "edge effect"/neointimal hyperplasia was in this trial sharp fall-off in radiation at the edges of the stents.