We studied different gas dilution techniques in order to validate an easy method for the measurement of functional residual capacity (FRC) in patients with acute respiratory failure (ARF) undergoing mechanical ventilation (MV). FRC was measured in duplicate using four different techniques in 22 adult patients. The reference technique was the bag-in-a-box (BB) with helium (BB He) which was compared with a BB with nitrogen (BB NO, with helium syringe (He S), and with N2 syringe N2 S) techniques. The mean values of FRC (in BTPS units) obtained with the different techniques were 1,108 mL ± 398 mL (BB He), 1,061 mL ± 418 mL (BB N2, 1,170 mL ± 431 mL (He S), and 1,050 mL ± 429 mL N2 S), the differences not being statistically significant. The linear regression equations were BB N2 = -42 + 0.99 BB He, He S=54 +1.00 BB He, and N2S= -66 + 1.00 BB He. The mean difference between duplicate determinations ranged between -1.8 mL and 14 mL. The techniques studied are useful and practical bedside procedures for the estimation of FRC in ARF patients undergoing MV. © 1988.