In patients with stable asthma, we assayed plasma proteins in the bronchoalveolar lavage fluid to obtain information on plasma exudation into the airways. Fourteen nonsmoking patients with asthma who were in a stable period of their disease and eight nonsmoking healthy volunteers were studied. The ratios of the concentrations of albumin, ceruloplasmin (CP), and α-2-macro-globulin (A2M) between blood and epithelial lining fluid were calculated (cQalb, cQCP, and cQA2M). The cQalb was increased in the patients (Mann-Whitney U test, p < 0.05). In 10 patients the bronchial hyperreactivity was assessed with histamine provocation tests. Significant relationships between the cQalb, cQCP, and cQA2M on the one hand and PC15 on the other hand were found (Spearman's rank correlation: r = -0.62, p < 0.05; r = -0.61, p < 0.05; r = -0.79, p < 0.01, respectively). Fourteen patients were treated with two inhalations of 200 µg glucocorticosteroids per day in a 3-month prospective study. Three of them were excluded from further study because of an intercurrent exacerbation of asthmatic symptoms during therapy. In the 11 patients with stable asthma, the cQalb and cQA2M decreased after treatment with inhaled steroids (Wilcoxon's matched pairs signed rank test, p < 0.03). Our results show that in patients with stable asthma, there is an increased plasma exudation into the airways, most likely caused by an increased respiratory membrane permeability. The plasma exudation correlated with the bronchial hyperreactivity to histamine, and it decreased after corticosteroid therapy.