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The determination of total lung capacity (TLC) by either the plethysmographic (pleth) or closed-circuit foreign gas (dil) method requires expensive and sophisticated equipment. Since hospitals usually have the facilities to perform chest roentgenograms, we wanted to ascertain if the radiologic method (rad) of BARNHARD et al. [1] could be used in lieu of the more sophisticated methods. Sixty-two men ranging from 21 to 67 years had TLC determined by the plethysmographic method, the closed-circuit foreign gas method in which rebreathing was continued until helium was uniformly distributed throughout the lungs, and the radiologic method. There were 21 normal men, 30 with a clinical diagnosis of chronic bronchitis and 11 with a radiologic diagnosis of emphysema plus chronic bronchitis. The mean value of TLC in the normal men showed no significant differences between the three methods and an excellent correlation between the pleth-rad and dil-rad. In the men with chronic bronchitis, there were no significant differences between the mean TLC of rad (7.166 +/- 1.217 1) and dil (6.704 +/- 1.067 1) of rad and pleth (7.545 +/- 1.165 1). The correlations (p < 0.001) were: dil-rad (r = 0.78) and pleth-rad (r = 0.77). In the men with emphysema, there were also no significant differences between the mean TLC of rad (7.166 +/- 1.217 1) and dil (6.704 +/- 1.067 1) or rad and pleth (7.545 +/- 1.165 1). The correlations (p < 0.001) were: dil-rad (r = 0.78) and pleth-rad (r = 0.77). In the men with emphysema, there were also no significant differences between the mean TLC of rad (8.789 +/- 1.207 1) and dil (7.831 +/- 1.041 1) or rad and pleth (9.403 +/- 1.735 1). The correlations were: dil-rad (r = 0.81, p < 0.01) and pleth-rad (r = 0.91, p < 0.001). This study suggests that in normal individuals and patients with chronic bronchitis or emphysema the radiographic method is a useful and reliable technique for determining TLC when elaborate equipment for measuring lung volumes is not available.