This study describes the histopathology and ultrastructure of bronchial mucosa in lung biopsies from two children with bronchial asthma in remission, and compares them with lung samples from two children who died in status asthmaticus. Light microscopy of all samples showed changes typical of bronchial asthma, e.g. mucus plugging, goblet cell hyperplasia, 'thickening of bronchial basement membrane', peribronchial smooth muscle hypertrophy and eosinophilic infiltration. Electron microscopy revealed that the mucus plugs consisted of moderately electron-dense floccular material containing degenerate epithelial cells, macrophages and cell fragments. The luminal surfaces of ciliated cells showed cytoplasmic blebs and abnormal cilia. Mast cells in various stages of degranulation were scattered between bronchial epithelial cells. The subepithelial hyaline layer, commonly referred to as "thickened basement membrane", consisted of collagen fibrils in plexiform arrangement. The basement membrane proper appeared intact. These electron microscopic changes, particularly the presence of mast cells and subepithelial collagen deposits, were also found in autopsy samples. This combined light and electron microscopic study shows that marked, possibly irreversible changes may be present in the lungs of patients with severe bronchial asthma, even when they are asymptomatic. These pulmonary changes could be the direct consequence of mast cell activation and the release of various mediators. No evidence of immune complex deposition was found.