The epithelial basement membrane and underlying collagen were examined by immunohistochemistry and electron microscopy in endobronchial biopsy specimens from 8 asthmatic and 3 control subjects. There was an excess of interstitial collagens beneath the basement membrane in the asthmatic subjects. There was no evidence of any epithelial contribution to this abnormality, nor was there any correlation with epithelial damage, disease duration, or severity. These findings contradict the long-held notion of basement membrane thickening in asthma and indicate that the subepithelial fibrosis is a result of fibroblast activation rather than bronchial epithelial cell dysfunction.