This study was designed to evaluate the efficacy of scintigraphy by using 111In-oxine labeled platelets for the detection of either intracardiac or intravascular thrombi. Fourteen cases of thrombi diagnosed or suspected on the basis of either angiography or two-dimensional echography were examined. They included four cases of left ventricular thrombi, three of left atrial thrombi, four of vascular thrombi and three of inflammatory disease. Platelet samples from the patients were labeled by 111In-oxine according to the method originated by Yui et al. The labeling efficiency was 72.4 +/- 9.8%. The platelets retained good function except for collagen aggregability. Hot areas of high quality in the scintigram were demonstrated in six cases including two with left ventricular thrombi, two with left atrial thrombi and two with vascular thrombi. These areas accorded well with the findings of angiography, echography or surgery. The removed thrombi had a layer of aggregated platelets on the surface. Platelet survival was shortened in a case of prosthetic valve endocarditis without a hot area, and was normal in four out of five cases including two with a hot area. It is concluded that this method is potentially predictive in detecting thrombi.