The tidal flow volume (TFV) loop ratios of (1) time to peak flow (tPTEF) to total expiratory time (tE) [tPTEF/tE] and (2) volume to peak flow (VPTEF) to expired volume (VE) [VPTEF/VE] are reported to decrease with age in early life, and to decrease in subjects with obstructive airways disease (OAD). However, the mechanisms behind these changes are not well known. Thus, we reanalyzed data from 24 healthy neonates (mean birthweight: 3.49 kg +/- 0.42 kg (SD)), 26 presently asymptomatic asthmatic children (age: 33 +/- 21 months), and 26 controls (age: 34 +/- 19 months) to elucidate what is responsible for the changes in these ratios in health and disease. Lung function was measured by TFV loops (SensorMedics 2600) at 1 hour of life and on the following day in the neonates, and before and after inhaled nebulized salbutamol (0.05 mg/kg) in the asthmatics and their controls. The observed decreases in mean tPTEF/tE and VPTEF/VE from 1 hour to 1 day of life (neonates) were entirely due to increased tE and VE, respectively secondary to a decrease in respiratory rate (P = 0.03). In asthmatics (young children), the decreased baseline tPTEF/tE and VPTEF/VE were due to lower tPTEF and VPTEF, with no significant differences in tE and VE in asthmatics and controls. The improved ratios in asthmatic children following inhalation of a bronchodilator were mainly due to increased tPTEF and VPTEF. Our observations point out the importance of evaluating both tPTEF and either tPTEF/tE or VPTEF/VE when attempting to differentiate between changes in ratios that are related to age versus changes that reflect underlying obstructive airways disease.