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 less than or equal to 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 RT 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 RT and Raw were not significantly different after aerosolized isoproterenol (P greater than 0.05). We determined the inverse relationship between height and RT with correlation coefficient r = -0.79 (P less than 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, RT utilized in this study is a rapid and reproducible approximation of Raw. It allows evaluation of RT over a wide range of frequencies including resonant frequency and requires less cooperation than body plethysmography.(ABSTRACT TRUNCATED AT 250 WORDS)