In the balloon catheters the redundancy of the deflated balloon produces 2 or 3 sort of thin wings. The presence of wings can reduce the catheter "pushability" or, in certain conditions, can determine tissue lesion (winging effect). Simulating a transeptal valvuloplasty of the mitral valve by technique of the 2 balloon in human fresh heart, we studied the winging effect over the interatrial septum in 12 hearts; in the first 6 the atrial septum was dilated with a 6 mm balloon (Group A), in the second 6 the atrial septum was dilated with a 10 mm balloon (Group B). The procedure was completed introducing consecutively 2 bigger balloons (15 + 20 mm). We suppose that the winging effect of the 2 bigger balloons (15 + 20 mm) could counter balance the theoretical advantage of a small balloon (6 mm). The atrial septal defect (ASD) after septal dilatation was 5.0 +/- 0.59 mm x 1.56 +/- 0.25 mm (long axis x short axis) in Group A and 6.53 +/- 0.35 x 2.16 +/- 0.39 mm in Group B (p less than 0.01). The final ASD (after introducing the 2 bigger balloons) was 7.04 +/- 1.06 x 2.36 +/- 0.57 mm in Group A and 7.03 +/- 0.18 x 2.16 +/- 0.32 mm in Group B (NS). Our data show that the winging effect can determine biological negative effects.