This study was designed to evaluate whether indobufen and ticlopidine can induce changes in the size of left ventricular thrombi and variations in the deposition of platelets on thrombus surface. Forty-seven patients with left ventricular thrombosis, who were not treated with antithrombotic drugs, were prospectively evaluated with <sup>111</sup>In-oxine platelet imaging and two-dimensional echocardiography. The first scintigraphic examination was negative in 15 of the 47 patients with left ventricular thrombosis, thus they were excluded from further evaluation. The remaining 32 patients with evidence of labeled platelet deposition on the thrombus were divided into three groups. Group 1 comprises 11 patients treated with different doses of ticlopidine: 6 with 250 mg/day, and 5 with 500 mg/ day. Group 2 comprises 12 patients who received 400 mg/day of indobufen. Group 3 comprises 9 patients who were not treated with antithrombotic drugs. All 32 patients underwent repeated <sup>111</sup>In-oxine platelet imaging and echocardiography 40 ± 11 days after the first examination. During treatment with ticlopidine, deposition of labeled platelets on the thrombus became absent in 2 patients (500 mg/day), and reduced in 5 (2 treated with 250 and 3 with 500 mg/day). A decrease of platelet deposition on the thrombus was also observed in 5 of the 12 patients receiving indobufen and in only 1 of 9 controls. With regard to thrombus dimensions, 1 patient treated with ticlopidine showed a decrease in thrombus size associated with a reduction of the scintigraphic activity. In conclusion, a decrease of the platelet uptake on the thrombus surface, without significant changes in the size, was detected in most patients during treatment with indobufen and ticlopidine. Only during high doses of ticlopidine was a complete break in the deposition of platelets observed.