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In order to evaluate the sensibility and specificity of the maximal treadmill exercise-stress test, a correlation was made, in fifty patients, of the resus of the test and the findings at selective coronary arteriography. None of the patients had received cardioactive drugs previous to the study. Among twenty-nine patients with proven coronary disease, three had false negative exercise tests. The remaining twenty-six cases had positive tests. The sensitivity of the method (reliability in identifying the presence of disease) was 89.7%. Twelve of twenty-one subjects with normal coronary arteries had negative exercise tests. In eight patients without coronary artery lesions, the exercise test was positive (false positive tests). An additional patient had an equivocal response and is included among the false positive responses. These nine-patients form a special group since they had severe heart disease, even though it was not due to atherosclerosis of the major coronary vessels. The specificity of the test (reliability in identifying the absence of disease) was 57.1%. However, in the absence of other heart disease, none of the subjects with normal coronary arteries had a false positive response. An analysis is made of the possible causes of these false negative and false positive responses. It was also shown in this study that the patients with coronary artery disease and positive treadmill tests had a definite functional aerobic impairment as well as a significant reduction in such indices of myocardial oxygen consumption as the heart rate and the double product (pressure-pulse), when compared to the subjects with normal coronary vessels and negative tests. In the group of patients with false positive responses, these parameters were not significantly different from the normals. The exercise-stress test protocol used in this study appears to have an adequate sensibility and an acceptable specificity.