Three patients were studied in whom percutaneous transluminal angioplasty (PTA) was attempted in stenotic saphenous vein bypass grafts. In two cases, large splits were present in the thick fibrotic intima of the vein grafts, accompanied by rupture of atherosclerotic plaques or hemorrhage into atherosclerotic plaques. Both cases required emergency coronary artery bypass surgery. In the third case, a vein graft failed to dilate after multiple attempts. Only small splits were present in that fibrotic intima. Two other patients were studied in whom coronary artery PTA was attempted. In both cases, laceration of a nonatherosclerotic segment of the artery occurred. Combined with our previous report, these observations indicate that PTA results in lacerations of atherosclerotic plaques, fibrotic intima, and otherwise normal vessel walls and that laceration is the mechanism of both successful dilation and serious complications.