- Background: The optimal management of coronary lesions involving side branches is not known. New devices are being developed for dedicated bifurcation stenting or improved side branch access or protection. Several of these devices are bare-metal stent (BMS) platforms. The risk of side branch compromise and outcome after standard BMS is not known. Methods: We evaluated the frequency of side branch involvement and the rate of side branch compromise in a retrospective analysis of 1,440 patients from three pooled historical BMS clinical trials. The impact of side branch compromise on in-hospital and nine-month clinical outcomes was assessed. Results: Side branches >2 mm in diameter were present within the stented segment in 39% of treated lesions. Compromise of a side branch occurred in 27% of 1,440 lesions, including 22% of side branches that were not diseased prior to stenting. Compromise was associated with increased in-hospital MI (13.5% vs. 7.6%, P = 0.001), a trend for increased nine-month clinically driven target vessel revascularization (TVR, 17.4% vs. 13.2%, P = 0.058), and increased nine-month target vessel failure (composite of cardiac death, target vessel MI or TVR, 29.7% vs. 20.2%, P < 0.001). Conclusion: At least moderate-sized side branches are involved frequently in lesions enrolled in routine clinical trials. Unexpected compromise of these side branches is common and is associated with increased in-hospital and nine-month adverse outcomes. © 2010 Wiley-Liss, Inc.