We assessed the accuracy of the respiratory inductive plethysmograph in the supine position to spirometry by the two-body position, least squares calibration and single-body position, isovolume calibration procedures. The comparison was carried out simultaneously in normal subjects breathing naturally and with voluntarily controlled abdominal or thoracic breathing, and in patients with COPD breathing naturally and with voluntarily controlled abdominal breathing patterns. In both groups, there was no significant difference in estimation of tidal volume between the 2 calibration procedures for the various breathing patterns. There was greater deviation from spirometric tidal volume values for both calibration methods in patients with COPD during abdominal than during natural breathing. In the normal subjects, agreement between the rib cage and abdominal partitioning of tidal volume for both calibration methods was good, but in the patients with COPD there was greater variability. In normal subjects, over a wide range of rib cage and abdominal compartmental contributions to tidal volume, either calibration procedure appears satisfactory. For patients with COPD, if large changes occur in the distribution of rib cage and abdominal contributions to tidal volume, then validation of respiratory inductive plethysmography to spirometry must be rechecked.