The sirolimus-eluting stent (SES) has dramatically reduced restenosis in patients with most types of coronary lesions, but bifurcation lesions remain a predictor of poor prognosis even in SES implantation. We aimed to determine the clinical outcomes of the left main trunk (LMT) side branches (SB) treated with a single SES strategy. SES implantation was successfully performed on 70 patients with LMT stenosis from August 2005 to August 2008. Of the 70 patients, 55 patients (59 SB) were treated with a single SES and a jailed SB. The 56 SB were divided into two groups according to percent diameter stenosis immediately following percutaneous coronary intervention (PCI) (Group 1: >50%; Group 2: <50%). Quantitative coronary angiography (QCA) data was evaluated. One year angiographic follow-up was performed on 23 patients (82.1%) in Group 1, and 21 patients (78.6%) in Group 2. Group 1 MLD was significantly larger at follow up than that post-PCI (0.9 ± 0.4 vs. 1.2 mm ± 0.6 mm, p < 0.01). In Group 2, MLD and percent diameter stenosis did not change significantly. SB revascularization was not performed on either group at follow-up. Clinical outcomes of moderate stenosis in LMT SB treated with a single SES were acceptable, and stenosis progression within 1 year was minimal. We concluded that patients who present moderate residual stenosis in LMT SB with no presentation of ischemic symptoms should be observed rather than re-stented.