T lymphocytes and eosinophils are important components of the inflammatory cell infiltrate in bronchial mucosa in asthma. Because activated lymphocytes migrate through the thoracic duct and the general circulation to remote glandular and mucosal sites, we initiated this study to evaluate pathological abnormalities and immunoreactivity for interleukin (IL) 3, IL-5, and granulocyte/macrophage colony-stimulating factor (GM-CSF) of intestinal mucosa in bronchial asthma. 15 asthmatic patients, 8 nonasthmatic patients with chronic obstructive pulmonary disease, 6 atopic nonasthmatic healthy controls, and 6 nonatopic healthy controls were studied. Duodenal biopsies were performed by endoscopy. A significantly increased number of intraepithelial lymphocytes and eosinophils and a significant accumulation of mononuclear cells (lymphocytes and mast cells) and eosinophils in the lamina propria were detected in asthmatics and atopic controls. Immunostaining with antibodies directed against IL-3, IL-5, and GM-CSF was positive in asthmatics and atopic controls, whereas no staining was observed in nonatopic controls and chronic obstructive pulmonary disease. Combined ultrastructural study and immunogold labeling demonstrated that IL-3, IL-5, and GM-CSF were localized in eosinophils and mast cells. Although devoid of gastrointestinal symptoms, asthmatics and asymptomatic atopics had duodenal pathological abnormalities mimicking those observed in the bronchial mucosa in asthma, suggesting that the whole mucosal immune system is involved in bronchial asthma.