Angina pectoris results from an imbalance between oxygen supply and demand in the subendocardium. The haemodynamic effects contributing to this imbalance have been studied in 10 patients with coronary artery disease. Myocardial oxygen demand was estimated from the tension time index (TTI), potential subendocardial flow from a diastolic pressure time index (DPTI), and the oxygen supply/demand ratio from (DPTI/TTI. With progressively increasing pacing rates up until induction of angina, no significant change in TTI was found whereas a significant fall in DPTI and DPTI/TTI occurred (P less than 0.001). During pacing runs with induction of angina DPTI/TTI reached a minimum value 5 s before,and at the onset of angina. No such relationship was seen for TTI or DPTI alone. A significant rise in LVEDP (P less than 0.05) and fall in dP/dt min (P less than 0.01) occurred at angina both contributing to a further reduction in DPTI and DPTI/TTI. Changes in DPTI/TTI may then reflect changes in the myocardial blood flow supply/oxygen demand ratio in the presence of coronary artery disease and haemodynamic changes before and at the induction of angina lead to a further reduction of this ratio.