The diagnosis of prosthetic valve thrombosis (PVT) may be difficult in cases without valve obstruction. The aim of this study was to evaluate the diagnostic accuracy of 111indium-labelled autologous platelet imaging in the detection of PVT. We studied 41 patients with 45 prostheses, mechanical (37) or biological (8), in the mitral (26) or aortic (19) site. Mean age was 55 +/- 13 years. All these patients experienced one or several thromboembolic events (mean: 2.3 per patient). The mean interval between the last embolic event and indium test (IT) was 32 +/- 25 days. Scintigrams were recorded in two views daily for 5 days. IT showed an abnormal platelet deposition on the prosthetic area in 24 patients. Platelet half-life was 3.8 +/- 1.0 days for patients with a positive IT and 4.6 +/- 0.6 days for those with a negative IT (P less than 0.01). Anatomical data were obtained in 10 cases (surgery nine, autopsy one) 16 +/- 5 days after IT. All these patients had PVT, and IT was positive in eight cases. In the other 31 medically treated patients, recurrent embolism occurred within 12 months after IT in four cases, three of whom previously had a positive test. We conclude that IT is a useful means for detecting PVT.