Background: An accurate evaluation of warm heart surgery cannot be limited to the assessment of the myocardial effects of warm blood cardioplegia but should also address the effects of systemic normothermia on the inflammatory response to cardiopulmonary bypass. A major component of this response is the endothelial adhesion of neutrophils, because it is linked to the release of cytotoxic compounds. This study was designed (1) to characterize the bypass- induced changes in the expression of neutrophil adhesion molecules (L- selectin and β2-integrins) and (2) to assess the influence of bypass temperature on these changes. Methods and Results: Twenty case-matched patients undergoing open-heart procedures were divided into two equal groups according to the core temperature during cardiopulmonary bypass: warm (33.4±0.3°C) or cold (27.1±0.4°C, P<.0001 versus warm). Arterial blood samples were collected before, during, and 30 minutes after bypass and processed for the expression of L-selectin and β2-integrins (CD11a, CD11b, and CD11c) with flow cytometry. Warm bypass was associated with an early and sustained upregulation of CD11b. In contrast, hypothermia resulted in a strikingly less pronounced CD11b upregulation during bypass. However, CD11b expression sharply increased thereafter so that 30 minutes after bypass, it was no longer significantly different between the two groups. Changes in CD11c expression grossly paralleled those described for CD11b. Neither CD11a nor L-selectin changed significantly from baseline values in either group. Conclusions: Clinical cardiopulmonary bypass is associated with a marked upregulation of the neutrophil CD11b and CD11c integrins. Hypothermia delays but does not prevent the increased expression of these adhesion molecules, which could consequently represent logical targets for interventions designed to blunt the neutrophil-mediated component of bypass-induced inflammatory tissue damage.