In 40 drug-free patients 2052 episodes of transient myocardial ischemia (28% painful) were recorded by 24-h ambulatory ECG. Pain was more frequently associated with longer duration of ischemia and greater ST segment shift and with the presence of ventricular arrhythmias within the ischemic episode. Heart rate always showed a growing trend during the ischemic episode. Circadian distribution of painless ischemic episodes showed a maximum peak at 2 a.m. while the most painful attacks occurred early in the morning (6 a.m.).