We have assessed bronchial mucous gland size in a group of 26 patients with severe chronic air-flow obstruction and hypoxemia from the NIH Nocturnal Oxygen Therapy Trial. These patients had their sputum volume assessed at least 4 times when free from exacerbations. Mucous gland size was assessed by the gland/wall ratio (Reid Index), by absolute gland area, and by the volume proportion of glands. The volume of sputum produced was significantly related to the volume proportion of mucous glands (Rho = 0.53, p less than 0.01) and to the absolute gland area (Rho = 0.49, p less than 0.05), but not to the Reid Index, (Rho = 0.35, p greater than 0.05). Volume proportion of glands can be easily and quickly measured using a computer-assisted digitizer, and it is the method of choice for measuring mucous gland size in this series. Neither the Reid Index nor the volume proportion of glands was related to the forced expiratory volume in one second (FEV1), and the measurements were not significantly related to each other. The amount of sputum produced was not related significantly to the FEV1 (Rho = 0.26, p greater than 0.05).