It is a common conception that the balloons used in percutaneous transluminal coronary angioplasty will inflate to a certain designated diameter and maintain this diameter over the wide range of inflation pressures. With use of an optical magnification system, the diameters of multiple angioplasty dilating catheters made of polyvinyl chloride (USCI) and polyethylene (Advanced Cardiovascular System, Inc.) were measured with designated balloon diameter sizes of 2.0 to 3.7 mm. All balloons had a linear increase in diameter over a range of pressures from 2 to 10 atm. The average increase in diameter over this pressure range was 15% (range 7 to 18%). Polyvinyl chloride balloons reached their specified diameter at less than 5 atm and exceeded it at higher inflation pressures. Polyethylene balloons reached their specified diameter at 8 to 10 atm, and were below specified diameters at lower pressures. There was slight variation in size and only a minimal difference in compliance between balloons of the same material and size. A given balloon achieved the same diameter at a given pressure even after multiple inflations and maintained a stable diameter during prolonged inflations.