Myoglobin, an oxygen-binding protein, is synthesized exclusively in striated and cardiac muscle, and is normally found in blood. Serum myoglobin determination has been used in the diagnosis of acute myocardial infarction. Experimental work has shown that myoglobin is released only after muscular necrosis. This prospective study included 101 patients: 62 with acute coronary insufficiency, 16 with acute myocardial infarction, and 23 controls. In all the patients with infarction the serum myoglobin levels were elevated. None of the controls showed serum myoglobin above normal. In patients with coronary insufficiency the peak serum myoglobin ranged from normal to 280 ng/ml. Half of all the patients with coronary insufficiency had a significant elevation of serum myoglobin (p less than 0.001). The obvious explanation of this finding is that myocardial necrosis to some extent develops in cases of so-called coronary insufficiency. Furthermore, this study confirms previous findings that serum myoglobin assessment constitutes a very early marker of myocardial damage.