In this study, we compared the amount of lung tissue irradiated when respiratory gating was imposed during expiration with the amount of lung tissue irradiated when gating was imposed during inspiration. Our hypothesis was that the amount of lung tissue spared increased as inspiration increased. Computed tomography (CT) image data sets were acquired for 10 patients who had been diagnosed with primary bronchogenic carcinoma. Data sets were acquired during free breathing and during breath-holds at 0% tidal volume and 100% tidal volume, and, when possible, at deep inspiration, corresponding to approximately 60% vital capacity. Two treatment plans were developed on the basis of each of the gated data sets: one in which the treatment portals were those of the free-breathing plan, and the other in which the treatment portals were based on the gated planning target volumes. Dose-mass histograms of the lungs calculated at 0% tidal volume were compared to those calculated at deep inspiration and at 100% tidal volume. Data extracted from the dose-mass histograms were used to determine the most dosimetrically beneficial point to gate, the reduction in the amount of irradiated lung tissue that resulted from gating, and any disease characteristics that might predict a greater need for gating. The data showed a reduction in the mass of normal tissue irradiated when treatment portals based on the gated planning target volume were used. More normal lung tissue was spared at deep inspiration than at the other two gating points for all patients, but normal lung tissue was spared at every point in the respiratory cycle. No significant differences in the amount of irradiated tissue by disease characteristic were identified. Respiratory gating of thoracic radiation treatments can often improve the quality of the treatment plan, but it may not be possible to determine which patients may benefit from gating prior to performing the actual treatment planning.