The assess whether the magnitude of exercise induced ST segment depression improves the predictive values of symptom limited exercise tests, and helps in the recognition of patients with more severe coronary heart disease, 90 consecutive patients with positive treadmill tests who also underwent selective coronary arteriography were reviewed. The predictive value improved progressively with the increasing ST depression and was most reliable in a select group of patients with normal electrocardiographic baseline who were not receiving digitalis (73% with ST depression greater than or equal to 1 mm to 100% with ST depression greater than or equal to 4 mm). The incidence of 2 and 3 vessel disease increased from 61% with ST depression greater than or equal to 1 mm in the overall population to 100% with ST depression greater than or equal to 4 mm in the select group, and the incidence of left main trunk lesions increased, respectively from 6 to 30%. The prediction of 2 and 3 vessels disease was found to be significantly greater when patients were dichotomized into those with ST depression greater than or equal to 4 mm compared to less than 4 mm. It is concluded that the magnitude of ST segment depression definitely improves the predictive values of exercise tests as well as the ability to recognize the patients with more severe disease. However, the markedly positive exercise tests cannot be utilized to accurately predict the presence of 2 or 3 vessel disease in individual cases unless ST depression attains 4 mm or more in patients with normal electrocardiographic baseline who are not taking digitalis. In this group, the ability to predict left main trunk lesion is approximately 30%.