['The risk of cardiovascular disease is significantly higher in patients with long-term uremia than in otherwise healthy adults. This is true even before patients proceed to dialysis, but the reason why cardiovascular risk is increased is unknown. Transvascular transport of lipids and other macromolecules in both large vessels and the microcirculation has been implicated in generation of cardiovascular disease.', 'To determine whether patients with long-term uremia have circulating factors that promote increased vascular permeability, we measured the effect of perfusing microvessels with uremic plasma in a non-mammalian model of vascular permeability measurement.', 'Perfusion of frog mesenteric microvessels with dialyzed normal plasma did not result in an increase in either hydraulic conductivity (Lp, permeability of the vessel wall to water) or oncotic reflection coefficient (sigma, permeability to macromolecules, particularly proteins). Perfusion with dialyzed uremic plasma resulted in a very significant increase in vascular permeability to both water (Lp increased 8.8-fold from 4.1 to 36.4 x 10(-7) cm x s(-1) cm H2O(-1)) and proteins (sigma reduced from 0.93 to 0.53).'] Does human uremic plasma increase microvascular permeability to water and proteins in vivo?