Intracardiac and arterial thrombi were examined by scintigraphy using In-111-oxine labeled autologous platelets. In 22 cases of myocardial infarction including six with ventricular aneurysms, four had positive findings of thrombi on imaging and detected also by echocardiography. All four had ventricular aneurysms. The so-called "moya-moya" echoes (fuzzy echoes) were demonstrated in two of these four cases. We encountered two cases with positive findings on imaging in 13 with mitral valve disease. These two had systemic embolic episodes after scintigraphic examination. "Moya-moya" echoes were detected in the left atrial cavity in four with negative findings on imaging. Positive images were obtained in two of three with acute arterial occlusive disease, and in both cases platelet deposition was observed in the proximal site of obstruction. Though thrombectomy was performed for one of these two cases, no thrombus was detected at the site of platelet deposition. After one month, re-examination revealed only negative findings in all sites in both these patients. In the six cases of aortic aneurysm, three had platelet deposition within their aneurysms, and surgery was performed for these positive cases, but one of them had no thrombus. Positive images were obtained in only one of seven patients with chronic arterial occlusive disease. Coagulation tests and platelet studies were investigated for patients with positive or negative platelet scans. Only the data of the thrombo-test showed a significant difference (97 +/- 9% vs 23 +/- 7%, p less than 0.001). Three cases of positive imaging became negative after anticoagulant therapy. We tried ECT for eight cases 24 hours after injection of In-111-oxine labeled platelets. Three cases showed clear images of thrombi, while the planar images could not detect them at an early stage. Therefore, we propose that ECT can be a useful technique for diagnosing intracardiac thrombi in early stage.