Total respiratory resistance (RT) was measured in 11 normal children, 2- 5.5 years of age (mean 4.16 years), and in 24 children with bronchial asthma, 2- 10.5 years of age (mean 6 years), by oscillating the respiratory system from 2- 32 Hz with a new microprocessor technique. Twenty-five children were ≤ 5 years of age. This technique was also compared with airway resistance (Raw) by body plethysmography both in terms of baseline values and response to bronchodilators. The absolute values for Raw are lower than R, with a mean percentage difference (±S.E.M.) of 30.3 ± 5.0%; however, good correlation between the two techniques was obtained (r=0.79). The changes of RTand Raw were not significantly different after aerosolized isoproterenol (P> 0.05). We determined the inverse relationship between height and RTwith correlation coefficient r=-0.79 (P< 0.01). In children, resonant frequency ranged from 6-24 Hz with a mean value of 16.57 ± 0.78 Hz. The forced oscillation method utilized clinically in this study provides an alternative to total body plethysmography or an esophageal balloon, which are technically difficult and preclude their routine use in small children. In conclusion, RTutilized in this study is a rapid and reproducible approximation of Raw. It allows evaluation of R, over a wide range of frequencies including resonant frequency and requires less cooperation than body plethysmography. It requires a relatively short time and allows objective assessment of obstructive airway disease and its responsiveness to bronchodilators in children under 5 years of age.