['To compare radial and femoral artery perfusion pressure during initiation and various stages of cardiopulmonary bypass (CPB).', 'Prospective study.', 'The cardiac center of a tertiary referral teaching institute.', 'Sixty consecutive patients of all ages undergoing a variety of cardiac operations.', 'Radial and femoral arterial pressures were measured in all patients on the same transducer, from the beginning to end of CPB.', 'Mean perfusion pressures on CPB measured at the femoral artery at 1, 5, 10, and 15 minutes of CPB were 38.4+/-3.6, 46.2+/-3.1, 49.7+/-3.9, and 52.8+/-4.1 mmHg and were significantly greater than the corresponding radial artery pressures (29.9+/-4.1, 35.3+/-6.1, 40.9+/-4.8, and 41.8+/-5.3 mmHg) (p < 0.001). At 30 minutes and 60 minutes of CPB, femoral artery pressures are higher (60.3+/-8.8 mmHg and 66.4+/-8.2 mmHg) compared with radial artery pressures (54.7+/-6.9 mmHg and 59.6+/-6.1 mmHg), but the difference is less significant (p < 0.05). On conclusion of CPB, mean femoral artery pressures (70.9+/-6.7 mmHg) are greater than mean radial artery pressures (67.6+/-8.1 mmHg) (NS).'] Are femoral artery pressures more reliable than radial artery pressures on initiation of cardiopulmonary bypass?