Reversal of the usual relationship between aortic and radial artery pressure can occur in patients following cardiopulmonary bypass. Radial systolic (and often radial mean) pressures were lower, relative to aortic pressure, after cardiopulmonary bypass than before bypass in all 18 patients studied. The systolic pressure difference (aortic minus radial) was large enough to be of clinical concern (12–32 mmHg) in 13 patients. The change persisted for 10–60 min, gradually returning toward normal. The change temporally was associated with warming at the end of cardiopulmonary bypass and lowered forearm vascular resistance. Relative forearm vascular resistance (x) predicted the systolic aortic minus radial pressure difference (y) by the equation y = −0.34x + 17 for all patients (r = −0.49,P< 0.001). The authors conclude that radial artery pressure does not accurately reflect central aortic pressure in the immediate postbypass period.