We assessed the incidence of clinical heart failure in patients with acute myocardial infarction admitted to a coronary care unit and treated with intravenous streptokinase. We compared 2 groups of patients: 1) treated group: patients with acute myocardial infarction admitted to the unit in the last 3 years and treated with intravenous streptokinase, following a protocol established previously. 2) CONTROL GROUP: patients with the same characteristics and selection criteria as for the treated group, admitted to the unit during the previous 2 years and conventionally treated, without thrombolytic therapy. We assessed, in both groups, the incidence of heart failure at the time of admission, at discharge and the total incidence in the unit, following the Killip and Kimball criteria. The total incidence of heart failure was higher in the control group than in the treated group (43.8 vs 19.1%, p less than 0.001). This difference was even greater when the comparison was made with the reperfused patients (43.8% vs 18%, p less than 0.001). Heart failure incidence at the time the patients were discharged from de unit was also higher in the control group (21.2% vs 4.3%, p less than 0.001). When we considered severe heart failure (III-IV Killip Group) we also observed a significant difference between both groups. In conclusion, the incidence and the severity of clinical heart failure were lower in patients treated with streptokinase than in those treated conventionally.