For the purpose of coronary angioplasty (PTCA) coronary stenoses over 80% usually can only be passed with very small catheters. We therefore developed an instrument with a very small tip-diameter and two balloons of different diameters arranged in line. In 9 cases primary success has been achieved and the degree of stenoses reduced from 89% to 22%. The gradient of 58 mm Hg was only 11 mm Hg after PTCA. When used with caution, the advantages of this catheter in high-degree coronary stenoses is obvious.