45 patients with uncomplicated myocardial infarction and on no cardiac drugs underwent an exercise test and 24-hour monitoring 53 (range 22-80) days after the acute event before entering a 4-week rehabilitation program. These same tests were repeated 21 days later. Severe ventricular arrhythmias were registered in 14 patients (31%) while monitored on tape, in only 1 patient during exercise. Arrhythmias were more common in patients with ST-depression during exercise (38%) and in patients with reduced work capacity (42%). Non-smokers had no severe arrhythmias. The rehabilitation program had no influence on these findings. Arrhythmias after myocardial infarction correlate with work capacity and myocardial ischemia.