The balloon expandable, stainless steel, flexible coil stent is a useful device for the management of acute or threatened closure after percutaneous transluminal coronary angioplasty. Appropriate sizing is important to optimize immediate results and reduce the risk of acute thrombosis and restenosis. The in vivo size at deployment of this stent at nominal inflation pressures has not been evaluated. Forty patients who received a single flexible coil stent (10 patients each for stent sizes 4.0, 3.5, 3.0, and 2.5 mm) were studied. The stents were deployed at 3-5 atmospheres. The actual stent size achieved by the stent during deployment was found to be significantly less than the nominal size, being 3.7 +/- 0.3, 3.2 +/- 0.2, and 2.8 +/- 0.2 mm for nominal stent sizes of 4.0, 3.5, and 3.0 mm, respectively (P < 0.005, P < 0.001, and P < 0.01). For a 2.5 mm stent, it was 2.4 +/- 0.2 mm (P = ns). Thus, for optimal results with this device, we recommend that vessels 2.0-2.4 mm in diameter be supported by a 2.5 mm stent, vessels 2.5-2.9 mm in diameter by a 3.0 mm stent, vessels 3.0-3.4 mm in diameter by a 3.5 mm stent, and vessels 3.5-4.0 mm in diameter by a 4.0 mm stent.