In this prospective, randomized, placebo-controlled trial the effect of 100 mg acetylsalicylic acid (ASA) once daily on the incidence, hematologic activity and embolic potential of left ventricular (LV) thrombosis was studied in 100 consecutive patients with a first anterior wall acute myocardial infarction (AMI). Patients were randomized to ASA or placebo less than 12 hours after onset of symptoms. Heparin, 5,000 IU subcutaneously twice daily, was given to all patients during immobilization. Echocardiography was performed less than 24 hours, 48 to 72 hours and 1, 2, and 12 weeks after AMI. LV thrombosis was detected by echocardiography in 30 (33%) of the 92 evaluable patients (15 patients given ASA and 15 given placebo). Indium-111 platelet scintigraphy was done in 17 of the 22 patients with an LV thrombus at the second week echocardiogram. Among 7 ASA-treated patients, 4 had positive images; among 10 placebo patients, 5 had positive images. LV thrombus resolution was noted in 3 of 9 patients with a positive scan and in 5 of 8 patients with a negative platelet scan. In 7 of 10 ASA-treated patients and 5 of 12 placebo-treated patients thrombus resolution was observed (difference not significant). Systemic embolism occurred in 2 patients, both given ASA, during the first week after AMI. Thus, low dose ASA has no effect on the incidence, hematologic activity and embolic potential of LV thrombosis in anterior wall AMI.