A simple and accurate airway evaluation method for infants remains elusive. Lung sound analysis is a non-invasive method of bronchial condition assessment not requiring the infant's cooperation. Airway inflammation and airflow limitation affect breath sounds, even in the absence of adventitious sounds. Recent developments in signal processing methods have improved the possibility of extracting physiologically and clinically relevant information from lung sounds. We developed a new technology for analyzing lung sounds involving an index, named ic700 (index chest wall at 700 Hz), which shows the intensity difference at 700 Hz. The ic700 can overcome the effects of body size and flow. Our method of lung sound recording involves a sound sensor fixed with tape on the chest wall for 20 seconds while the children, without requiring a mouthpiece and/or nose-clip, are able to breathe quietly. We showed that ic700 is for infants who are unable to perform forced breathing. Moreover, ic700 could evaluate the effect of ICS treatment and airflow limitation in asymptomatic asthmatic children, even including infants. We suggest that ic700 reflects breath sound generation and transmission in asthmatic children. We found that ic700 is a useful index for the management and daily monitoring of asthma in infants.