The thickness of the basement membranes of bronchial epithelial cells varies under various pathological conditions. It has been reported that this membrane is thickened in patients with bronchial asthma. By light microscopy, this parameter was measured in biopsy specimens of bronchial mucosa obtained by fibre-optic bronchoscopy. These specimens were obtained from 171 patients who had undergone bronchial biopsy between 1984 and 1994. It was demonstrated that the thickness of the basement membrane of bronchial epithelial cells was weakly correlated with the patient's age, when thickness was examined in patients with lung cancer (r = 0.242, P = 0.0268). The basement membranes in patients with bronchial asthma (8.193 +/- 1.362 mu, mean +/- SEM) were significantly thicker than those without bronchial asthma (5.145 +/- 0.233 mu) (P = 0.0180, Mann-Whitney's U-test). In addition, it is noteworthy that the basement membranes in patients with diabetes mellitus (7.217 +/- 0.753 mu) were also significantly thicker than those without diabetes mellitus (4.968 +/- 0.235 mu) (P = 0.0038, Mann-Whitney's U-test). The background or underlying pathophysiology in such patients should be studied further, with attention directed towards the thickness of the bronchial basement membrane in bronchial biopsy specimens.