Experimental studies of transluminal angioplasty in atherosclerotic animal models as well as in human postmortem arteries all have shown that splitting of the atheromatous plaque occurs during angioplasty. Histologic sections of human arteries that were studied after successful angioplasty done in vivo also have shown splitting of the atheromatous plaque. The split may extend down to the internal elastic membrane. As the angioplasty balloon becomes fully inflated, the elastic media and adventitia stretch to conform to the outer diameter of the expanded balloon. Damage to medial cells may occur and may be important in keeping the artery dilated after the balloon is deflated and withdrawn. The "healing" process of the atheromatous plaque after angioplasty is poorly understood. Whether there is metabolic dissolution of atheromatous material or whether there is fibrous retraction of the atheromatous plaque against the dilated arterial wall is not known. Further studies are needed to elucidate the late changes after angioplasty.