Our initial studies indicate that children who develop post- operative capillary leak syndrome (CLS) following cardiac surgery with cardiopulmonary bypass (CPB) can be distinguished based on their pre-operative level of circulating cytokines an adhesion molecules. We tested flow cytometric analysis of surface antigen expression as a potential assay for risk assessment of CLS. 24th preoperative blood samples were stained with monoclonal antibodies for the adhesion molecules ICAM-1, LFA1, MAC1, (beta) -integrin, activation markers CD25, CD54, CD69, HLA- DR, CD14 or CD4. Cells were measured on a dual-laser flow cytometer calibrated with microbeads. Antigen expression was detected as mean fluorescence intensity. The data indicate, that neutrophils of CLS patients express preoperatively higher levels of LFA1 and monocytes higher levels of HLA-DR and activation markers thus are in a state of activation. This could in combination with surgical trauma and CPB lead to their additional stimulation and migration into sites of inflammation and induce postoperative CLS. It is planned to set up a Flow-Classification program for individual risk assessment. By discriminate analysis over 80 percent of the patients were correctly classified. Our preliminary study indicates that flow cytometry with its low samples requirements and rapid access of the results could be a powerful tool to perform risk assessment prior to pediatric open heart surgery.