The effects of placebo were studied in 150 patients (135 men, 15 women) aged 42 to 75 years with stable exertional angina pectoris, using multistage graded exercise testing. Treadmill exercise, using on-line computer analysis of the electrocardiogram, was performed after a basal period, during which time the patients had no treatment for 2 weeks, and after 2 weeks of placebo therapy. Mean exercise time during no treatment was 6.0 +/- 0.2 minutes and during placebo was 6.1 +/- 0.2 minutes (difference not significant). Similarly, time to development of 1 mm of ST-segment depression of 4.0 +/- 0.2 minutes without treatment was 4.1 +/- 0.2 minutes after 2 weeks of placebo therapy (difference not significant). Placebo failed to show any effect on rest or maximal heart rate or on maximal ST-segment depression. It also failed to increase exercise tolerance or to improve other objective indexes of effort-induced myocardial ischemia in both single-and double-blind protocols in patients with stable exertional angina pectoris. Therefore, placebo control of antianginal drug trials that use exercise testing for evaluation of effect is unnecessary and can be omitted.