We compared 11 patients with upper airway obstruction (obstruction at or proximal to the carina) to 22 patients with chronic obstructive pulmonary disease and to 15 normal subjects utilizing spirometry, lung volumes, airway resistance, maximal voluntary ventilation, single-breath diffusion capacity, and maximal inspiratory and expiratory flow-volume loops. Four values usually distinguished patients with upper airway obstruction: (1) forced inspiratory flow at 50 percent of the vital capacity (FIF50%) less than or equal to 100 L/min; (2) ratio of forced expiratory flow at 50 percent of the vital capacity of the FIR50% (FEF50%/FIF50%) larger than or equal to 1; (3) ratio of the forced expiratory volume in one second measured in milliliters to the peak expiratory flow rate in liters per minute (FEV1/PEFR) larger than or equal to 10 ml/L/min; and (4) ratio of the forced expired volume in one second to the forced expired volume in 0.5 second (FEV1/FEV0.5) larger than or equal to 1.5. The last ratio can be determined with a simple spirometer.