T lymphocytes are a major component of bronchial inflammatory processes in asthma. Because lymphocytes have the ability to migrate from one mucosal site to another, we initiated this prospective study to demonstrate mucosal abnormalities of the digestive barrier in asthma. To establish this we studied intestinal permeability in a group of 37 patients with asthma (21 allergic and 16 nonallergic) by measuring chromium 51–labeled ethylenediaminetetraacetatic acid (CrEDTA) urinary recovery. The results were compared with those obtained in a group of 13 nonasthmatic patients with chronic obstructive pulmonary disease and 26 healthy control subjects. Urinary recovery of CrEDTA was significantly higher in patients with asthma (2.5% ± 1.95%) than in patients with chronic obstructive pulmonary disease (1.16% ± 0.48%) and healthy control subjects (1.36% ± 0.14%). There was no significant difference in intestinal permeability between patients with allergic asthma (2.94% ± 2.4%) and those with nonallergic asthma (1.92% ± 0.9%). Intestinal permeability was not correlated with the severity of asthma as measured by FEV₁. Similarly, intestinal permeability did not significantly vary according to Aas score or steroid treatment. Serum IgE values and eosinophil blood count were not correlated with intestinal permeability. Intestinal permeability was evaluated sequentially in seven patients with asthma (4 allergic and 3 nonallergic) with a mean interval of 7.6 months (range, 2 to 13 months) and did not significantly change. Our results support the hypothesis that a general defect of the whole mucosal system is present as a cause or a consequence of bronchial asthma. (J ALLERGY CLIN IMMUNOL 1996;97:1173-8.)