The occurrence of restenosis following percutaneous transluminal coronary angioplasty is prevented or inhibited using a photopheresis treatment method, In accordance with the photopheresis treatment method, a photoactive compound such as 8-methoxypsoralen is administered to the patient's blood or affected tissue, or some fraction thereof, in vitro or in vivo using conventional administration routes. A portion of the patient's blood or affected tissue is then treated (preferably, extracorporeally...