BACKGROUND: A study was undertaken to evaluate the reliability of a digital tracheal sound analyser (ELENS-DSA) in predicting nocturnal changes in airways resistance in asthmatic patients. This device allows continuous measurement of the proportion of the time occupied by wheezing (Wh%). METHODS: Nocturnal polygraphic studies with simultaneous continuous monitoring of tracheal sounds and airways resistance were performed in seven patients with nocturnal asthma. In order to evaluate the possible bias in wheezing estimation, each tracheal sound recording was passed through the automatic analyser and simultaneously monitored with earphones by an experienced observer. RESULTS: The device detected audible wheezing with an optimal sensitivity and specificity of 70%. Snoring was a minor cause of the relatively poor characteristics of the system. A close correlation (p < 0.001) between Wh% and airways resistance was observed only in those patients with the highest increase in resistance; when the results of all the subjects were pooled the correlation observed was poor. The predictive value of Wh% in detecting changes in airways resistance during 10 minute intervals was lower than 70%. The positive and negative predictive values of Wh% were raised to 79% and 83%, respectively, for 30 minute intervals. CONCLUSIONS: The ELENS-DSA system is a relatively crude means of detecting wheezing and assessing bronchoconstriction quantitatively. However, it is able to detect accurately nocturnal bronchoconstriction for 30 minute intervals. This finding, along with the fact that the monitoring is non-invasive, suggests that it may be a promising tool, especially for patients during sleep.