Inflammatory lipid mediators, PAF and leukotrienes (LTs), are thought to have an important role in biocompatibility in hemodialysis. PAF, LTB4 and LTC4 were studied both in controls (n: 12) and in 11 children on regular hemodialysis (150 minutes) with cuprophane dialyzers. Blood samples were collected initially (0'-precapillary), at first minute (1'-postcapillary) and at one hour after the hemodialysis sessions (210'-venous). Presence of LTs and high levels of PAF in 0' samples compared to levels in controls and significant increases in 1' samples suggested the alterations in PAF and LTs likely originated from the peripheral leukocyte activation. In 210' samples, PAF and LTs levels were decreased but still higher than the levels in 0' samples. This study suggested that PAF and LTB4 may be the control elements in biocompatibility in hemodialysis with cuprophane membranes, and demonstrated that the effects of activation last until the following session.