The degree of dose perturbation produced by lung‐equivalent material exposed in a 45‐MV x‐ray beam was determined as a function of beam size, chest wall thickness, and lung thickness. It was found that the dose levels in the distal portion of the lung were increased above that measured in a water phantom. However, a significant portion of the lung received less dose than that observed at the same depth in a water phantom. An examination of the dose levels beyond lung and in the chest wall was also made.