The intake of (n-3) long-chain PUFA is associated with a decreased risk of fatal myocardial infarction. Whether this effect is attributable to the effects of docosahexaenoic acid 22:6(n-3) (DHA) on vascular function, particularly at intakes <1 g/d, is unknown. We report a randomized, double-blind, crossover, placebo controlled trial of 0.7 g DHA/d as a purified algal derived triacylglycerol (1.5 g/d) vs. placebo (1.5 g olive oil/d) on vascular function and biochemical indices of endothelial dysfunction in 38 healthy men and women, aged 40-65 y. Each treatment phase lasted 3 mo, separated by a 4 mo washout period. Supplementation increased the proportion of DHA in erythrocytes lipids by 58%, compared with placebo. Arterial compliance and endothelium independent and dependent responses, plasma concentrations of C-reactive protein, soluble thrombomodulin, E-selectin, von Willebrand factor antigen, and urinary microalbumin and isoprostane excretion were unaffected by treatment. Diastolic blood pressure decreased by 3.3 mm Hg (95% CI -6.1 to -0.6; P = 0.01). Heart rate tended to be 2.1 beats/min lower after DHA treatment than after the placebo period (P = 0.15). The results indicate that a moderate increase in the daily intake of DHA to ~0.7 g DHA lowers diastolic BP but does not influence indices of endothelial function or arterial stiffness in the short term.