An acoustic gas analyzer was developed to measure helium during the closing volume (CV) test. It is based on the principle that altering the composition of a gas mixture will change the velocity at which sound is transmitted through the gas. The change in transit time for an acoustic burst across an in-line (breathe-through) cell is measured as a function of He concentration in the expired air during the CV test. Although CO2 and H2O vapor affect the signal, so it is not specific for He, the system provides good records of CV. A comparison of measurements in 21 normal subjects demonstrated no differences for CV measured in the same breath by the bolus technique using the acoustic gas analyzer and by the resident gas method using a nitrogen analyzer. Differences in closing volumes measured by the He bolus technique and the resident gas method might exist in patients with some kinds of pulmonary disease, and the suggestion is made that CV measurements by the two methods might assist in describing pulmonary alterations in patients with bronchopulmonary disease.