Leukotrienes (LTs), the end products of the eicosanoid pathway released during inflammation, are markers of polymorphonuclear cell and monocyte activation. The present study focused on the possibility that 5-lipoxygenase (5-LO), the key enzyme for LT synthesis, was involved in the interaction between blood and the hemodialysis (HD) membrane. 5-LO gene expression was examined by reverse transcriptase polymerase chain reaction (RT-PCR) in samples of mononuclear cells isolated from peripheral blood withdrawn at the start and at 15 min of HD from 10 chronic HD patients, 5 treated with Cuprophan and 5 with polymethylmethacrylate (PMMA) membrane. An increased 5-LO gene expression was detected at 15 min in 4 of 5 patients using the Cuprophan membrane but in none of the 5 PMMA treated patients. Our results showed for the first time that the interaction between blood and the HD membrane upregulates 5-LO messenger ribonucleic acid (mRNA).