OBJECTIVE To document sleep apnea in the acute phase of myocardial infarction. If apnea occurs in patients with myocardial infarction, hypoxemia induced by apnea might exaggerate insufficiency of oxygen supplied to the damaged myocardium. METHODS Prospective controlled study. METHODS Critical care unit of a teaching hospital. METHODS Forty-nine patients, average age 64 yrs (range 49 to 91). RESULTS Patient measurements were recorded on a polygraph using an apnea-monitor, pulse oximeter, pulmonary artery pressure monitor, and an ECG. All of the patients observed showed frequent apneic episodes. The apnea was especially frequent when the cardiac index was low. Capillary oxygen saturation of less than 90% (suggesting systemic hypoxia) was observed in 21 patients concomitantly with apnea. Occasionally, arrhythmias followed these episodes (premature supraventricular contractions [n = 10], premature ventricular contraction [n = 4], and ventricular tachycardia [n = 2]). CONCLUSIONS This study suggests that sleep apnea is common in the setting of acute myocardial infarction. It may be a factor predisposing to, or even causing, sudden death in patients with acute phase of myocardial infarction.