Thirty patients with threatening myocardial infarction were treated with intravenous isosorbide trinitrate. Eight patients had increasingly severe angina, 6 had de novo crescendo angina, 3 had Prinzmetal angina and 13 had signs of impending extension of a previous infarct. In all cases the anginal attacks occurred spontaneously. The drug was administered in association with a beta-blocker or a calcium antagonist. The initial dosage was 33 mcg/min and dosage adjustments ranged from 16 to 130 mcg/min. the main duration of treatment was 3.6 days. Pain was controlled in all patients. Anginal attacks ceased completely and permanently in 24, but the remaining 6 became isosorbide dinitrate-dependent and could only be weaned by aortocoronary bypass. The effects on the drug on heart rate and blood pressure remained moderate and never interfered with dosage adjustments. Coronary artery angiography was performed without any trouble in 25 patients, 21 of whom underwent myocardial revascularization by venous grafts.