BACKGROUND: Coronary stent is one of effective and well-accepted treatments for coronary artery diseases. Stenting of coronary lesions, however, sometimes involves the coverage of relatively large side branches located in the vicinity of the target lesion. Serial angiographic changes in side branches of stented coronary segments were analyzed to determine the incidence and clinical outcomes of side branch occlusion. METHODS: Serial angiographic findings of 51 patients who had total 60 side branches originating from the stented segments including large branches more than one millimeter in diameter were analyzed. Side branches were divided into two types:type A (> or =1 mm in diameter, with ostial narrowing>50%), type B (> or =1 mm in diameter, with ostial narrowing>50%). Side branch occlusion was defined as development of total occlusion or morphologic changes from type B to A or reduction of TIMI flow more than I after stenting. RESULTS: After stent deployment, 4 out of 60 side branches occluded totally and 2 out of 4 side branches regained luminal patency with the improvement of TIMI flow (type A, TIMI II) on follow-up coronary angiography. Another one branch which showed type B, TIMI flow II changed into type A, TIMI flow I. There were no clinical cardiovascular events associated with acute side branch occlusion. On follow-up coronary angiogram, side branch occlusion developed in 20 (33.3%) side branches. The incidences were significantly related with in-stent restenosis (11/17, 64.7% in group with retenosis vs. 9/34, 26.4% in group without restenosis, p=0.003). CONCLUSIONS: Acute side branch occlusion can develop in a few stented patients without any clinical deteriorations and delayed side branch occlusion may be associated with in-stent restenosis.