A scintigraphic technique using indium-111 labelled platelets to detect left atrial thrombi was used in 28 patients, 14 of whom had mitral valve disease and 14 combined valve disease. Imaging was performed in the anterior, right anterior oblique (45 degrees), and left lateral views on the day of injection and thereafter at one or two day intervals for a maximum of four days. When scintiphotos obtained in two or three views 72 or 96 hours after the platelet injection showed "hot spot areas" within the left atrial pool and indium-111 activity in these areas did not decrease with time they were interpreted as positive for thrombi. Of 28 patients, seven had positive platelet images by this criterion; of these, three underwent surgery and were found to have left atrial thrombi. One patient died, and a thrombus was found at necropsy. The remaining 21 patients had normal scintiphotos; of these, seven had no thrombi at operation and one had false negative images. The diagnostic accuracy of platelet scintigraphy by this criterion of positivity in the 12 patients in whom surgical or postmortem confirmation of thrombi could be obtained was 92%. These results indicate that this technique is a promising method for detecting active left atrial thrombi.