92 significant stenoses of the left main stem artery (LCA) underwent revascularisation procedures by aorto-coronary venous bypass grafting. Three groups of patients were identified. Group I comprised 11 stenoses of the LCA associated with lesions of at least three other coronary vessels and a subtotal stenosis of the LCA. These patients underwent systematic prophylactic intra-aortic balloon counter-pulsation (IACP) when they were taken off cardiopulmonary bypass. Group II comprised 47 patients with stenosis of the LCA and at least three coronary vessels. IACP was required in 6 cases in whom difficulty was experienced at the end of cardiopulmonary bypass, or after haemodynamic decompensation. Group II comprised 33 stenoses of the LCA which were isolated or associated with lesions of 1 or 2 vessels. The operative risk increased with the severity of the coronary lesions associated with LCA stenosis. 6 p. 100 deaths occurred in Group III, compared to 21 p. 100 in Group II (p = 0.06). the use of IACP before the onset of possible haemodynamic decompensation improved the operative prognosis of stenosis of the LCA associated with lesions of at least three coronary vessels. Mortality in Group I was 0 p. 100, compared to 21 p. 100 in Group II (p = 0.09). Prophylactic IABP also seemed to decrease the number of perioperative infarctions: Group I, 16 p. 100, Group II, 36 p. 100, but p > 0.01. analysis of global mortality (12 cases) showed that 50 p. 100 patients (6 cases) had ejection fractions of less than 0.50. In agreement with other authors, we found that systematic IABP improved the operative prognosis in stenosis of the LCA. However, we do not advocate its use in every case. A selection of high risk patients seems a reasonable objective. The severity of the coronary lesions associated with LCA stenosis is statistically a good criteria of selection. The incidence of left ventricular hypokinesis with an ejection fraction less than 0.50 in the deceased patients, is a factor in favouring the use of prophylactic IACP in left ventricular dysfunction.