Lung function tests are normally performed in the upright position, whereas anesthesia is usually administered with the patient in the supine position, and occasionally in other postures. We therefore compared forced vital capacity (FVC), forced expiratory volume in 1 s (FEV1), functional residual capacity (FRC), and ribcage contribution to ventilation by respiratory inductive plethysmography in 13 conscious healthy male volunteers, sitting and in four horizontal positions used during anesthesia. Forced vital capacity and FEV1 were similar in all positions, except for a significant mean increase in FVC of 300 mL (SD 213) when sitting compared with when supine (P less than 0.001). The mean decrease in FRC was 806 mL (SD 293) between the sitting and supine positions (P less than 0.001). A significant increase in FRC occurred (252 mL, SD 329, P less than 0.01) when supine subjects raised their arms above their heads as required for computed tomography. Functional residual capacity in the prone and lateral positions was significantly larger than in the supine position (mean change 350 mL, P less than 0.001), but was still some 450 mL less than in the sitting position. Mean ribcage contribution was similar in all horizontal positions (32%-36%), whereas supine values were significantly different from those of the sitting position (mean 70%, SD 11, P less than 0.001). In conclusion, the various horizontal postures studied have no effect on FVC, FEV1, or ribcage contribution to ventilation. However, FRC in the prone, lateral, and arms-up positions is on average 250 mL larger than in the supine position, an observation that may affect gas exchange during anesthesia in these positions.