The measurement of functional residual capacity (FRC) has assumed new importance in the diagnosis of acute respiratory failure. However few reference values exist for FRC in supine positions for both sexes. We measured the FRC in 100 healthy subjects with the helium dilution closed circuit method in the sitting and supine positions. There were 50 women aged 20 to 63 years and 50 men aged 22 to 65 years. Fifty five subjects were smokers (25 women and 30 men) but no significant differences were found between FRC of smokers and nonsmokers. The FRC always decreased when the subject changed from the sitting to supine position. In this study, the correlation coefficients between FRC and height were statistically significant for both sexes, the influence of age and weight on FRC being negligible. Our results provide useful reference values for FRC in the supine position.