Forty-five patients presenting with unstable angina having 70 p. 100 stenosis of the left anterior descending artery judged acceptable for coronary bypass surgery were randomly allocated, using a table of random numbers, for medical (21 patients) or surgical treatment (24 patients). There were no significant differences between the two groups with regards to age (53 +/- 10 years for the medical group; 55 +/- 9 years for the surgical group), the length of follow-up (55 +/- 26 vs 61 +/- 28 months), left ventricular end diastolic volumes (87 +/- 27 vs 84 +/- 18 ml/m2) or ejection fraction (62 +/- 8 vs 59 +/- 11 p. 100). There were no deaths in the medical group; two patients developed uncomplicated myocardial infarction 19 days and 7 months after coronary angiography, respectively. 5 patients had recurrent angina and were referred for surgery. This operation of second intention did not pose any special problems. 6 of the 14 patients with stenosis of the LAD before the origin of the first septal artery had complications (infarction in 1 case, recurrent angina in 5 cases). In the surgical group, 1 patient died in the immediate postoperative period, of resistant cardiac arrhythmia; 2 patients developed uncomplicated peroperative myocardial infarction; 21 patients had no complications at all. The surgical patients were heparinised in the immediate postoperative period and anticoagulant therapy was continued with oral vitamin K antagonists for 6 months to 1 year, followed in some cases, by platelet antiaggregant therapy. 20 patients in this group underwent maximal exercise stress testing which was negative in 19 cases.(ABSTRACT TRUNCATED AT 250 WORDS)