The authors have studied the behaviour of ubidecorenone (Co Q10) in the acute phase of myocardial infarction in 24 patients, 19 male and 5 female, mean age 56.8 +/- 3.3. Ubidecorenone level was determined on admittance, after 48 hours and on the 7th and 30th days. A significant decrease was observed from the first to the 3rd day (mean values 0.90 +/- 0.18 microgram/ml vs 0.72 +/- 0.22, p less than 0.01). Thereafter a progressive rise was observed, but at the 30th day mean values were still below the basal ones. No significant differences were observed between patients treated with fibrinolytic agents and those not so treated, nor between those in whom reperfusion was obtained and the others. Nor was there a proven correlation with changes in creatinkinase. The behaviour of ubidecorenone may be associated with increased consumption for metabolic needs and increased destruction in scavenger action, and also to a lesser extent to decreased production due to lower food intake.