Increased free-radical production leading to oxidative stress may contribute to the development of cardiovascular complications in haemodialysis patients. The ferroxidase activity of caeruloplasmin forms an important component of antioxidant defences in body fluids. The aim of this study was to assess ferroxidase activity in haemodialysis patients. Venous blood was collected from 83 haemodialysis patients immediately prior to and after dialysis and from 52 healthy controls. Immunoreactive caeruloplasmin was measured by rate nephelometry, and ferroxidase activity determined by measuring loading of ferrous iron onto iron-free transferrin. A significant reduction in ferroxidase activity was observed in dialysis patients when compared with controls (37 ± 1.20 and 46 ± 1.14 mU/l, respectively; p < 0.001). Following dialysis, ferroxidase activity rose significantly to 41 ± 1.16 mU/l, with a significant difference still remaining between control and patient ferroxidase activity (p < 0.005). Immunoreactive caeruloplasmin was found to be similar in all groups (before dialysis 0.40 ± 0.07 g/l, after dialysis 0.39 ± 0.07 g/l, control 0.42 ± 0.09 g/l: p = NS). A significant difference in caeruloplasmin-specific activity was therefore observed between predialysis, postdialysis and control samples (97 ± 2.31, 105 ± 1.74 and 112 ± 1.51 mU/g; p < 0.001, p < 0.01, respectively). Ferroxidase activity of caeruloplasmin is impaired in renal failure. Inhibition of caeruloplasmin ferroxidase activity in dialysis patients may contribute to increased oxidative stress in these patients.