Reproducible and profound (greater than 2 mm) ST-segment depression during exercise testing in patients with coronary heart disease is associated with multivessel involvement. In these patients, coronary-artery bypass surgery has been recommended even when symptoms are absent. However, there are few long-term follow-up data regarding the prognosis when such patients are treated medically. Among 212 men with coronary-artery disease in whom profound ST-segment depression could be reproduced with exercise, 142 who had no other type of heart disease and were not receiving digitalis drugs had a mean ST-segment depression of 2.9 mm. Follow-up has lasted an average of 59 months: 11 patients have died (annual mortality, 1.4 per cent), and nine have had bypass operations (1.3 per cent per year). Survival correlated with exercise tolerance but not with degree of ST depression, peak heart rate, or peak blood pressure during exercise. We conclude that such ST-segment depression is not associated with a poor prognosis. There is rarely a need to resort to cardiac surgery; medical management is highly successful and associated with a low mortality.