Of 340 patients undergoing aorto-coronary surgery over a period of 6 years, the 136 cases in which there was a minimum follow-up of more than 4 years were analysed from the standpoint of the results. Overall amount annual postoperative mortality was low, less than 2,5%, even though the majority of patients undergoing surgery suffered from rest angina and had double or triple coronary stenoses. Analysis of prognostic parameters reveals the importance of the quality of the left ventricule, whilst age, sex, risk factors, the site of the bypass, the number of stenoses and the type of angina had little or no statistical influence on postoperative mortality. Six years after operation, there remained 66% of excellent results and 60% of the patients were able to work. One hundred and forty eight follow-up coronary arteriograms were carried out (including 33 repeated). During the first year after operation (184 by-passes examined) the percentage of permeability of the grafts was 84,8%. Later (86 bypasses examined) this fell significantly to 74,4% but 86,3% still had at least one permeable by-pass. Repeat late studies after four years showed the stability of the result. Investigation into the causes of postoperative infarctions ans analysis of the results in terms of the number of by-passes carried out in relation to the number of coronary stenoses, indicate that multiple by-passes should be avoided when technical conditions are difficult, with a narrowed coronary network and of little functional value.