Trapidil (triazolopyrimidine), a platelet-derived growth factor antagonist, is a potential inhibitor of intimal proliferation after percutaneous transluminal coronary angioplasty (PTCA). To study its efficacy, 72 patients were randomized to receive Trapidil (600 mg/day orally for 1 week before PTCA and for 4 to 6 months after PTCA; n = 36) or aspirin and dipyridamole (aspirin, 300 mg/day, and dipyridamole, 150 mg/day; n = 36). At entry, both groups were comparable with regard to age, sex, dilated vessels, severity of pre-PTCA stenosis, residual stenosis after PTCA, and prevalence of coronary risk factors. Repeat coronary angiography was performed 6 months after PTCA. Restenosis, defined as the loss of at least 50% of the gain in luminal diameter accomplished by dilation, was present in seven patients (19.4%) in the trapidil group and 15 patients (41.7%) in the aspirin-dipyridamole group (p less than 0.05). The progression of stenosis in patients with less than 30% residual stenosis was significant in both groups. Furthermore, in the patients with residual stenosis of more than 30%, progression of stenosis was less in the trapidil group than in the aspirin-dipyridamole group. Thus trapidil was useful in preventing intimal proliferation after PTCA, especially in patients with more than 30% residual stenosis after PTCA.