The role of the collateral circulation less than 6 hours after the onset of acute myocardial infarction (MI) was evaluated in 34 consecutive patients without previous MI. There were 19 patients with and 15 without collaterals. The group was subdivided into those with nonjeopardized collaterals (group A, 14 patients) and those with jeopardized collaterals (group B, 5 patients), and the group without collaterals into those with partially obstructed coronary arteries (group C, 5 patients) and those with totally obstructed coronary arteries (group D, 10 patients). These groups had similar sites of coronary stenoses and MI. Eleven of 14 collaterals in group A were poor, but MI mass measured by peak creatine kinase (CK) was smaller in group A than in group B (p less than 0.01) or group D (p less than 0.01), and cardiac function was significantly better in group A than in group D (cardiac index, p less than 0.05; stroke index, p less than 0.01; ejection fraction, p less than 0.01; regional wall motion, p less than 0.01). Group C was not statistically different from group A in myocardial function and CK. Group B was similar to group D in MI mass and cardiac function (cardiac index, stroke index, ejection fraction and regional wall motion). Thus, patients with nonjeopardized collaterals and those with partially obstructed coronary arteries had less myocardial damage and better cardiac function than did those with jeopardized collaterals and those with totally obstructed coronary arteries. A nonjeopardized collateral circulation may play a role in limiting MI mass and preserving myocardial function in the early stages of acute MI.