The respiratory flow is a good indicator of sleep-related breathing disorders. Common praxis is to use a pneumotachograph as the golden standard for flow measurement. However, it does not have to be necessarily the best possible test device for long-term condition, because the device is very uncomfortable and rarely suitable for measurement during sleep. A computer-based method to determine the respiratory flow, called ThorAKUSTIK, yielded a highly positive correlation between the calculated flow out of the tracheal breath sound and a measured flow signal. In order to avoid noise interference due to a breath-mask or a pneumotachograph, in this study we applied the ThorAKUSTIK-method to lung sound which was measured at the back of 18 subjects and investigated the correlation between the calculated flow and the measured flow by a pneumotachograph. The new method showed a highly positive correlation (r = 0.89 and 0.90). Additionally we examined the use of an accelerometer signal to distinguish between inspiration and expiration. In this case we got high correlation coefficients of r = 0.87 and 0.88 between the calculated and measured airflow as well.