A total of 197 polyethylene balloons of 2 to 4 mm diameter were inflated to 1 to 10 atmospheres to determine the variations of size with respect to pressure of inflation. A linear relationship was observed between diameter and pressure of inflation. The slope of this relationship represents the compliance. Balloons of the same diameter have almost identical compliances. The compliance of the balloon increases with size. The specified diameters are obtained with pressures of less than 6 atmospheres. However, the variations of size of a given balloon are important. This study shows that attention should be paid to the calibration of balloon catheters before angioplasty to avoid the risks of over-dilatation. New balloons made of therephtalane polyethylene are much less compliant and do not present this type of problem.