Scintigraphy with 111In-labeled autologous platelets was performed in 20 patients with suspected venous thrombosis and/or pulmonary embolism. The platelets accumulated in venous thrombi in 6 of 7 patients (86%) with positive findings on impedance plethysmography or contrast venography; all 6 were receiving intravenous heparin. In 11 patients with pulmonary embolism diagnosed by ventilation-perfusion imaging or pulmonary angiography, platelet scintigraphy showed embolic uptake only in the one patient not on full-dose heparin. These findings suggest that scintigraphy with 111In-platelets is a promising noninvasive technique for detection of deep venous thrombosis. Heparin does not appear to block localization of labeled platelets in venous thrombi, but may inhibit their adherence to pulmonary emboli.