Lesions in side branches of arteries having undergone percutaneous transluminal coronary angioplasty: a histopathologic study. Percutaneous transluminal coronary angioplasty (PTCA) may cause occlusion in side branches. No histologic studies, however, have been made on side branches of the arteries in which PTCA has been performed. A histologic study was therefore made to explain the effect of PTCA on side branches. Histologic specimens were prepared by serial step sectioning from 15 side branches of 10 autopsied cases that had undergone PTCA. The results of examination by light microscope were as follows: (1) Stenoses due to PTCA were seen in seven branches (46.7%). (2) The stenoses were classified into three types: (a) stenosis due to blocking of the orifice of a side branch by the disrupted portion of the intima of the main artery (one branch); (b) stenosis due to medial dissection of the main artery or further dissection occurring even in the side branches (three branches); and (c) stenosis due to fragmentation of the internal elastic lamina of the main artery accompanied by proliferation of smooth muscle cells even in the side branch (three branches). It is now clear that stenosis is caused in side branches long after PTCA. Extra care is required when major side branches exist in the portion where this procedure is to be performed. 