Smaller reference vessel diameter is a recognized determinant of in-stent restenosis. The SIRIUS 2.25 trial was a prospective, nonrandomized study including 100 patients (mean age 63.4 years; 64% men, 40% with diabetes mellitus) assessing the safety and efficacy of the 2.25-mm sirolimus-eluting Bx Velocity stent in patients with de novo native coronary lesions. Using propensity score matching for gender, diabetes mellitus, left anterior descending artery target vessel, lesion length, and reference vessel diameter, the outcomes were compared with historical control groups (angioplasty and Palmaz-Schatz stent arms from the STRESS/BENESTENT I/II trials and the Bx Velocity bare metal stent arm from the RAVEL and SIRIUS trials having a reference vessel diameter <3 mm). Use of the 2.25-mm sirolimus-eluting Bx Velocity stent was associated with a high rate of procedural success (97%) and a low rate of in-hospital major adverse cardiac events (2%). The primary end point, 6-month in-lesion binary angiographic restenosis, occurred less frequently in patients treated with the 2.25-mm sirolimus-eluting Bx Velocity stent than in each of 3 historical controls (16.9% vs 30.6%, p = 0.12; 36.5%, p <0.001; 45.9%, p <0.001, respectively). This translated into lower rates of 6-month target lesion revascularization in the 2.25-mm sirolimus-eluting Bx Velocity stent group (4.0% vs 15.0% in each of 3 control groups, p = 0.01 to <0.001). By multivariate analysis, in-lesion binary restenosis was predicted by multiple implanted stents (odds ratio 10.4, p = 0.002). Four of 13 patients who developed restenosis (30.8%) had a diffuse pattern of restenosis. In the long lesion tertile (mean lesion length 19.5 mm), the in-lesion binary restenosis rate was 27.6%. In conclusion, use of the 2.25-mm sirolimus-eluting Bx Velocity stent was safe and provided favorable 6-month clinical outcomes. Use of multiple stents (in longer lesions) was an independent predictor of in-lesion restenosis.