We prospectively evaluated nine patients with cough from postnasal drip for evidence of extrathoracic upper airway obstruction. Patients compared before treatment to normal control subjects had physiologic evidence of extrathoracic upper airway obstruction; their mean FIF50%/FEF50% and FIF25-75%/FEF25-75% ratios of 0.88 and 0.98 were significantly less than the values in control subjects of 1.28 and 1.37 (p less than 0.001). With specific therapy, postnasal drip decreased, cough disappeared and upper airway obstruction physiologically and physically resolved in all patients. We conclude that: 1) when postnatal drip is causally associated with cough, flow-volume loops can provide objective documentation of this clinical association; 2) flow-volume loops can be used as an objective method in comparing the efficacy of different therapeutic agents for cough due to postnasal drip; and 3) normal predicted values of extrathoracic airway function should not include measurements from patients who have recently recovered from cough associated with postnasal drip.