Considerable confusion exists regarding the frequency spectrum of the normal lung sound, especially the components in the lower frequency range. Frequencies of peak intensity varying from 10 to 200 Hz have been reported by different laboratories. A component of musculoskeletal sound could contribute to the inspiratory lung sound, but this has never been assessed. This study attempted to separate the sound made by the lung from that of muscular contraction by frequency analysis of the sounds occurring during inspiration and preinspiration and postinspiration open-glottis breath-holds in 4 normal subjects. The data showed that at frequencies below 200 Hz, the musculoskeletal component increased as the lung sound component decreased. At 50 Hz and below, the sounds of inspiration and postinspiratory breath-holding were almost indistinguishable. It is concluded that musculoskeletal noise seriously contaminates what is usually considered to be lung sound.