The immediate and long-term patency of intermediate size side branches was assessed by serial coronary angiography in 47 patients with 48 lesions to determine whether the presence of these side branches (1-2 mm in diameter) is unsuitable for Palmaz-Schatz stent implantation. Coronary angiography was performed at baseline, after conventional balloon angioplasty, immediately after stent implantation, at 1 week, and at 6-month follow-up. Sixty-eight lesion-associated side branches that were 1-2 mm in diameter, 11 with branch ostial stenosis (Group A) and 57 without ostial stenosis (Group B) were studied. After stent implantation, 9 (13%) branches became totally occluded and coronary flow deteriorated in 13 branches (19%). The incidence of side branch occlusion during the procedure in group A was greater than in group B (55% vs. 12%; p < 0.005). One (2%) patient suffered persistent chest pain, but no procedure was complicated by Q-wave myocardial infarction or significant elevation of creatine kinase concentration. Flow improved in 82% of the occluded side branches after 1 week and in 90% after 6 months. These results suggest that the presence of intermediate size side branches is not a contra-indication to Palmaz-Schatz stent implantation.