Postprandial lipaemia is known to cause endothelial dysfunction, but its underlying mechanism is still under debate. The present study was undertaken to investigate the effects of postprandial lipaemia on endothelial dysfunction and oxidative stress. We measured plasma glutathione peroxidase (GSH-Px), an antioxidant enzyme, and the urinary excretion of 8-epi-prostaglandin F2α (8-PGF2α), a free radical-catalysed product from the oxidative modification of arachidonic acid, in 16 healthy subjects (mean age, 30±5 years) without major coronary risk factors. Plasma high-sensitive C-reactive protein, soluble intercellular cell-adhesion molecule-1 and vascular cell-adhesion molecule-1 were also measured. High-resolution ultrasound was used to assess the flow-mediated vasodilatation (FMD) of the brachial artery. Blood and urine samples were collected before and 2, 4 and 6 h after a standard high-fat meal (3677 J, containing 50 g of fat). Serum triacylglycerol (triglyceride) increased and FMD decreased significantly after a high-fat meal. Plasma GSH-Px significantly decreased from 27.2±12.3 µg/ml to 25.7±11.8 µg/ml (P=0.022) 2 h after the meal, and urinary excretion of 8-PGF2α significantly increased from 1286±1401 pg/mg of creatinine to 2197±1343 pg/mg of creatinine (P=0.014) at 4 h after the meal. However, there were no significant changes in the levels of high-sensitive C-reactive protein and adhesion molecules after a high-fat meal. In conclusion, endothelial dysfunction was observed after consuming a high-fat meal and is associated with augmented oxidative stress manifested by the depletion of serum antioxidant enzymes and increased excretion of oxidative modification products.