We studied the efficacy of stress thallium scan in detecting restenosis after primary successful PTCA. There were 21 patients with angina pectoris and 16 patients with previous myocardial infarction. The sensitivity and the specificity of stress thallium scan in detecting restenosis were superior to those of stress electrocardiography or chest pain complained in follow-up period. In multi-vessel disease, we could assess the vessel developing restenosis more easily than stress electrocardiography. Initial thallium %UPTAKE RATIO was significantly improved after PTCA but redecreased in patients developing restenosis. In patients showing residual redistribution despite of not developing restenosis, we could judge vessel patency by gradually improving initial %UPTAKE RATIO after PTCA. Thus, stress thallium scan proved to be useful in detecting restenosis after PTCA.