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Coronary angioplasty was performed on 14 high-risk patients supported with venoarterial partial cardiopulmonary bypass. Hemodynamic, metabolic and physiologic parameters were monitored to assess the effect of cardiopulmonary support in conscious patients. Cardiopulmonary support caused a decrease in systolic (45 ± 17 to 27 ± 14 mm Hg, p < 0.001), diastolic (23 ± 12 to 14 ± 8 mm Hg, p < 0.005) and mean (29.7 ± 13.2 to 18 ± 9 mm Hg, p < 0.001) pulmonary artery pressures. Aortic systolic (129 ± 18 to 106 ± 17 mm Hg, p < 0.001), mean (89 ± 19 to 84 ± 19 mm Hg, p < 0.05) and pulse (64 ± 17 to 37 ± 16 mm Hg, p < 0.00001) pressures also decreased. Heart rate and aortic diastolic pressures were unchanged. End-systolic wall stress (122 ± 48 × 103 to 96 ± 44 × 103 dynes/cm2, p < 0.001) and left ventricular end-diastolic diameter (5.7 ± 0.8 to 5.5 ± 0.9 cm, p < 0.05) were reduced during partial cardiopulmonary bypass. After initiation of cardiopulmonary support, normal lactate extraction across the coronary circulation was diminished or converted to lactate production (38 ± 23 to 2 ± 29%, p < 0.005). There was a marked reduction in hematocrit (41 ± 4 to 28 ± 5%, p < 0.0001) and platelet count (259,000 ± 57,600/ml to 145,900 ± 46,000/ml, p < 0.0001) after bypass. Cardiopulmonary bypass successfully supported all patients during balloon inflation, for an optimal angioplasty result. During balloon inflation, 6 of 8 patients with interpretable electrocardiograms had ST shifts suggestive of ischemia and 1 developed ventricular tachycardia. During balloon inflation, pulse pressure decreased from a mean of 38 ± 15 to 25 ± 15 mm Hg (p < 0.01). It is concluded that cardiopulmonary support provides excellent systemic support but induces cardiac anaerobic metabolism in patients with severe coronary artery disease. Myocardial ischemia when coronary blood flow is interrupted during balloon inflation is still observed despite effective cardiopulmonary support.