95 angina patients surgically treated by aortocoronary saphenous vein by-pass, have been studied by comparing the factors limiting the exercise (i.e. muscular exhaustion, angina, electrocardiographic changes), the total work performed, the O2 consumption and the heart rate-arterial pression product. After surgery the incidence of positive effort tests (effort angina and/or ecg evidence of myocardial ischemia) decreased from 79% to 27,4% (p less than 0,001), and the average amount of work performed increased from 2718 to 3504 Kgm (p less than 0,01), the MVO2 from 13,9 to 15,6 cc/min/Kg. (p less than 0,01) and the heart rate-arterial pression product from 206 to 243 (p less than 0,001). The patients who preoperatively were more invalidated (that is able to performe a lasser amount of physical work) presented a more pronounced improvement in comparison with those who had a greater tolerance to exercise. It seems therefore that for these latter patients the bypass grafting surgery should be justified only if a significant increase of life expectancy and reduction of myocardial infarction incidence could be definitely proved. The exercise performance improvement after surgery was statistically significant only in the patients with post infarction or with unstable angina not in those with chronic angina. The perioperative infarction (present in 8,4% of cases) did not show any unfavorable correlation with the result of the late postoperative effort test.