Serum lactate dehydrogenase isoenzyme are separated 5 fractions (LD1-LD5) in normal serum by agarose electrophoresis. An unusual band on the cathodic side of LD5 isoenzyme has been found on lactate dehydrogenase electrophoresis is of serum, called LD6, and the presence of which signifies a poor prognosis for the patients of myocardial infarction. In recent, we have experienced LD6 in two patients with myocardial infarction. Cardiac arrest was noted within 24 hours after exhibition of LD6 extra band and they did not recover from their illness.