Transient ischaemic ST segment changes were studied in 296 consecutive patients with coronary heart disease attending hospital for coronary arteriography. They underwent two channel, frequency modulated ambulatory monitoring for 24 hours. During this time 221 episodes of transient ST elevation (n = 56) or ST depression (n = 165) with a horizontal deviation of at least 1 mm lasting at least 1 min were found in 70 patients (23.6%). Only 34% of episodes were associated with pain. The duration of the episode, the heart rate at the beginning of the episode, or the extent of ST deviation were not related to the occurrence of pain. Episodes of ST elevation were of significantly shorter duration, occurred significantly more often during the early morning, and at significantly lower heart rates than episodes of ST depression. The considerable overlap between the characteristics of episodes of ST elevation and ST depression suggests that in many instances a combination of factors is responsible for transient ischaemic ST segment changes.