The relationship between transnasal pressure and nasal flow is markedly curvilinear during tidal breathing in man, and there is poor agreement among the results of various methods used to define this characteristic with a single number. We used Rohrer's equation (P = K1 V + K2 V2), where P = pressure and V = flow, and calculated values for K1 and K2 from 474 nasal inspiratory pressure/flow curves obtained from 34 human subjects by a standard method of posterior rhinometry. Nasal airway inspiratory resistance at an air flow of 0.4 liter per sec (NAIR0.4) was also calculated. Rohrer's equation was found by regression analysis to fit each curve well (0.86 less than r less than 0.9999; mean, 0.983). There was a strong correlation between NAIR0.4 and K2: NAIR 0.4 = 0.91 K2 + 0.39 (r = 0.97). Nasal congestion induced in 7 normal subjects with histamine resulted in larger changes in K2 and NAIR0.4 than K1. Patients given an intranasal corticosteroid aerosol (beclomethasone dipropionate) in a double blind crossover trial showed symptomatic improvement in nasal congestion (P less than 0.01) and significant decreases in K2 (P less than 0.02) and NAIR 0.4 (P less than 0.05), but no change in K1 (P greater than 0.2).