Left ventricular thrombosis (LVT) is a frequent complication in Acute Anterior Myocardial Infarction (Anterior AMI) (40-60%) associated with an increased risk of systemic embolism (3-15%). IV heparin anticoagulat therapy in the first 24h of AMI, reduces thrombi formation in 34-48%. The objective of this study was to demostrate that subcutaneous (SC) heparin in high doses is as effective as intravenous (IV) heparin in preventing thrombi formation after an anterior AMI and to compare the rate of thrombohemorraghic events in the two schemes of treatment. This is an analytic, prospective, comparative, experimental and study in 33 patients, who were hospitalized randomized with acute anterior myocardial infarction, in first 24 hours, in the Emergency Room of Hospital San Juan de Dios (Bogota) between february 1993 and december 1995. Nineteen patients were randomly assigned to receive IV heparin and 14 patients to receive SC heparin at doses of 12.500 UI b.Ld. The level of anticoagulation was regulated tritated to maintain the JYIT 1.5 to 2 times the control. Each patient had echocardiographic examination at the end of the first week, to determine the presence of left ventricular thrombosis (LVT). The patients were followed during three months looking for complications related to anticoagulant therapy. It was found that the incidence of LVT in both groups was low (5.26% IV heparin scheme vs 7.1% SC heparin scheme), without statistically significant differences in complication rate and mortality. This study suggests that the effectiveness in preventing LVT in the post AMI period is similar in IV and SC heparin schemes, if a good anticoagulation level is warranted.