Plasma protein exudation into the airways is an important pathophysiological event in asthma. The effect of 12 wk of treatment with inhaled fluticasone propionate (FP; 250 μg twice a day) or salbutamol (Sb; 400 μg twice a day) on plasma protein leakage was compared in a double-blind, randomized parallel-group study of 30 patients with asthma. Primary outcomes were plasma protein leakage and size selectivity of the blood–airway lumen barrier, cell differentials in BAL fluid, and bronchial responsiveness to histamine (PC20histamine). Two independent procedures to account for the effect of variable dilution of BAL on the levels of albumin (Alb) and α2-macroglobulin (A2M) in BAL fluid consisted of correction based on urea levels and on the application of the relative coefficient of excretion [RCE = ([A2M] in BAL fluid/[A2M] in serum)/([Alb] in BAL fluid/[Alb] in serum)]. In the FP group a significant decrease was found in the A2M level and the RCE, and in the percentage of eosinophils in BAL fluid. The PC20histamine increased significantly (mean increase, 2.4 doubling doses), whereas PC20histamine decreased in the Sb group. Differences between groups were significant except for the decrease in eosinophils. We conclude that 12 wk of FP (250 μg twice a day) decreased the permeability of the blood–airway lumen barrier, in particular for high molecular weight proteins.