Background: In provisional side branch intervention strategy, it is important to predict the side branch occlusion after main branch stent implantation. However, incidence and predictors of side branch slow flow after drug-eluting stents (DES) implantation are not well-known. Methods: Patients with provisional DES implantation for bifurcation lesions were consecutively enrolled. Lesions were divided into slow flow (< TIMI 3 flow) and normal flow groups according to side branch TIMI flow grade after main branch DES implantation. Results: A total of 210 lesions (ICPS type 1: 49%, LAD-Dg: 63%, Cypher: 71%) in 200 patients (age: 63years, male: 64%, stable angina: 45%) were included. Slow flow in side branch was occurred in 18 lesions (8.6%) after main branch stent implantation. There were no differences in clinical characteristics between the 2 groups. True bifurcation lesions (ICPS type 1,4) were more frequent in a slow flow group (94% vs 56%, p<0.0001). Slow flow group had more severe pre-intervention side branch stenosis (minimal luminal diameter (MLD) 0.6짹0.3 vs 1.1짹0.6 mm, p< 0.001). Stent type was not different between 2 groups. In a multivariate analysis, pre-intervention side branch MLD was a single independent predictor (OR=0.03, 95% CI: 0.003-0.367) of side branch slow flow after main branch DES implantation (included variables: stent type, DM, hypertension, hyperlipidemia, diagnosis, bifurcation type, bifurcation angle, plaque location, main and side branch reference diameter, MLD). Lesions with lowest tertile of side branch MLD (< 0.71mm) had 6 times higher risk of side branch slow flow (95% CI:1.6-24.9). Lesions with lowest tertile of side branch MLD and true bifurcation lesion had 25% risk, whereas lesions without those characteristics had only 1% risk of side branch slow flow. Conclusion: Pre-intervention side branch MLD was the single independent predictor of side branch slow flow after main branch DES implantation. However, lesions with small MLD and true bifurcation type require protection before main branch intervention due to high risk of side branch closure.