Factors precipitating nocturnal myocardial ischaemia were investigated in 10 patients with frequent daytime and nocturnal angina pectoris. Eight patients had fixed obstructive coronary artery disease or a low exercise threshold or both before the onset of ischaemia. Two patients had variant angina with normal coronary arteries and negative exercise tests. During sleep the electrocardiogram, electroencephalogram, electro-oculogram, electromyogram, chest wall movements, nasal airflow, and oxygen saturation were continuously measured. Forty two episodes of transient ST segment depression were recorded in the eight patients with coronary artery disease and 26 episodes of ST segment depression and elevation in the two patients with variant angina and normal coronary arteries. All episodes of ST segment depression in the former group of patients were preceded by an increase in heart rate as a result of arousal and lightening of sleep, bodily movements, rapid eye movement sleep, or sleep apnoea (one episode). In contrast, in the variant angina group no increase in heart rate, arousal, or apnoea preceded 23 of the 26 episodes of ST segment change. Thus increase in myocardial oxygen demand was important in precipitating nocturnal angina in patients with coronary artery disease and reduced coronary reserve. In the patients with coronary spasm these factors did not often precede the onset of nocturnal myocardial ischaemia.