Ten serial pathological cross sections at 1 mm intervals of both the left anterior descending artery at the site of a percutaneous transluminal coronary angioplasty and of the circumflex artery in the untreated stenotic area were studied at necropsy in a patient who died immediately after the procedure. The extent of calcification and atheroma were similar in both branches. Intimal or medial splitting, desquamation, and plaque fracture were present in the left anterior descending artery. No typical pathological findings were seen in the circumflex artery. This study suggests that the original stenotic lumen may have been enlarged as a result of plaque splitting.