Fifty patients undergoing elective coronary revascularisation were prospectively randomised to receive either a leucocyte-depleting or a control filter inserted into the arterial line of the cardiopulmonary bypass (CPB) circuit. The concentration of exhaled nitric oxide (NO) was measured 15 min before and 30 min after CPB using a real-time chemiluminescence analyser (Logan Research, Northampton, UK). The baseline rate of exhaled NO production was 2.14±0.83 ppb/s in the control group, and 2.58±0.53 ppb/s in leucocyte-depleted group (p = 0.17). Following CPB, the mean rate of exhaled NO production in the control group had increased by 1.51±0.45 ppb/s to 3.65±0.81 ppb/s and in the leucocyte-depletion group had increased by 1.05±0.45 ppb/s to 3.64±0.62 ppb/s. The increase in exhaled NO production was significantly lower in the leucocyte depleted group (p = 0.002), indicating that leucocyte depletion suppressed the increase in exhaled NO production seen following CPB.