OBJECTIVE To document the trend in arterial hypoxaemia and electrocardiographic abnormalities on the second to sixth nights after acute myocardial infarction. METHODS Nineteen consecutive patients with acute myocardial infarction who were monitored continuously during the night (minimum 2300-0700) with a Holter tape recorder and a pulse oximeter. Fifteen patients were monitored for five nights, one patient for four nights, one patient for three nights, and two patients for two nights. RESULTS Five patients had > 30 episodic oxygen desaturations of > or = 5% during the nights of monitoring and many patients had episodes with oxygen desaturations to < 80% ranging from 46% to 61% (from 7/15 to 11/18 patients) during the nights of monitoring. Constant hypoxaemia was found in 11-13% (2/15) of the patients. Simultaneous episodic hypoxaemia and episodic tachycardia was seen in 9/17 (52%) patients on the second night, 11/18 (61%) on the third, 7/15 (46%) on the fourth, 8/15 (53%) on the fifth, and 5/15 (33%) on the sixth night. Simultaneous episodic hypoxaemia and ST deviation was seen in 5/17 (29%) patients on the second night, 3/18 (16%) on the third, 4/15 (26%) on the fourth, in no patients on the fifth, and in 3/5 (20%) on the sixth night. Simultaneous occurrence of episodic hypoxaemia and arrhythmias (supraventricular, ventricular ectopy, and atrioventricular blockade) was seen in 5/17 (29%) on the second night, 4/18 (22%) on the third, 4/15 (26%) on the fourth, 2/15 (14%) on the fifth, and in no patients on the sixth night. Overall, simultaneous occurrence of episodic hypoxaemia and electrocardiographic abnormalities (episodic tachycardia, ST deviations, and arrhythmias) was seen in 11/17 patients (64%) on the second night, 13/18 (72%) on the third, 10/15 (66%) on the fourth, 8/15 (53%) on the fifth, and 7/15 (46%) on the sixth night. One patient who died of cardiogenic shock had simultaneously occurring episodic hypoxaemia and nonsustained ventricular fibrillation on the night before she died. CONCLUSIONS Episodic and constant hypoxaemia are common during the first week after acute myocardial infarction. Episodic hypoxaemia was associated with electrocardiographic abnormalities in most patients. Thus, episodic nocturnal hypoxaemia may be particularly detrimental to the infarcted myocardium in the early phase after infarction; special attention should therefore be directed towards oxygenation in this group of patients.