Geometric analysis of the chest roentgenogram allows calculation of estimated gas volume. If roentgenograms are made on individual patients or in epidemiologic studies in which relatively expensive nitrogen meters or body plethysmographs are not available, the thoracic roentgenogram provides an economical alternative. This study compares two roentgenographic methods of estimating total lung capacity in 154 subjects who also had total lung capacity estimated by the 7-minute nitrogen-washout technique or by total-body plethysmography. Results with the roentgenographic techniques were closely comparable with each other and with those of the gas-dilution and total-body plethysmograph methods in normal subjects. The planimetric method consistently estimated higher total lung capacity than the elliptic method in men and women with total lung capacity of 7.75 liters or less. In patients with severe obstructive lung disease, the roentgenographic methods were more closely comparable with body plethysmography, which is believed to be the most accurate, whereas the nitrogen-rinsing and similar gas-dilutional methods tended to underestimate the true volumes. The roentgenographic techniques also proved to be reliable in a small group of patients with varying degrees of pulmonary fibrosis.