Background/Aims: Degranulation of polymorphonuclear leukocytes (PMN) during hemodialysis (HD) is usually assessed by measuring degranulation products. However, this process might also be estimated by the assessment of cell surface markers. In this study, the relationship between the expression of PMN degranulation markers (CD63 and CD66b) and the release of degranulation products [myeloperoxidase (MPO) and lactoferrin (LF)] was investigated during clinical HD in order to evaluate cell surface markers as a useful index of PMN degranulation. Methods: The expression of CD63 and CD66b on PMN and the release of MPO and LF were investigated in 10 chronic HD patients, during both heparin (HD<sub>hep</sub>) and trisodium citrate anticoagulation (HD<sub>cit</sub>), in a randomized order. Samples were drawn from both the efferent and afferent lines of the dialyzer at 0, 7.5, and 180 min. Results: During HD<sub>hep</sub> at first passage, a major increase in MPO (from 158 ± 32 to 448 ± 177 µg/l, p = 0.001) and LF (from 134 ± 52 to 260 ± 120 µg/l, p = 0.01) was found across the dialyzer, whereas marked changes were not observed during HD<sub>cit</sub>. The expression of CD63 and CD66b increased across the dialyzer during both anticoagulation modalities, but was only significant in the case of HD<sub>hep</sub> (CD63: mean fluorescence intensity from 247 ± 61 to 331 ± 118, p < 0.01; CD66b: mean fluorescence intensity from 340 ± 76 to 434 ± 103, p = 0.01). During HD<sub>hep</sub> a correlation was noted between the degranulation products and markers of both azurophilic and specific granules (MPO and CD63: r = 0.35; p < 0.01; LF and CD66b: r = 0.39, p < 0.01). Significant differences in the expression of CD63 and CD66b between HD<sub>hep</sub> and HD<sub>cit</sub> were not observed. When analyzing the combined data for both HD<sub>hep</sub> and HD<sub>cit</sub>, no correlation was observed between degranulation products and markers. Conclusion: Our data suggest that the measurements of cell surface markers may not be a reliable indicator of the degree of HD-induced PMN degranulation.