It is well established that unlike normal subjects patients with cervical cord transection have an increase in VC when changing from the seated to the supine posture. To investigate the mechanism of this paradoxical increase, we measured static lung volumes in both the seated and supine posture in 14 consecutive patients with tetraplegia (C4-C7) and in 4 patients with paraplegia (Th4-Th7). The increase in VC in the supine compared with the seated posture was (mean +/- SE) 0.41 +/- 0.07 L (16.0% of the seated value) in the tetraplegic subjects and 0.40 +/- 0.01 L (11.2% of the seated value) in the paraplegic subjects (p less than 0.001). However, TLC in all subjects was 0.28 +/- 0.05 L smaller in the supine posture (p less than 0.001), thus indicating that the larger VC in this posture is related to a reduction in residual volume (RV) rather than to an increased mechanical advantage of the diaphragm. The reduction in RV in the supine posture was consistent, averaging 0.72 +/- 0.06 L (29.1% of the seated value) in the tetraplegic subjects and 0.62 +/- 0.21 L (37.6 percent of the seated value) in the paraplegic subjects (p less than 0.001). Inflating blood pressure cuffs at the bases of the legs prior to the assumption of the supine posture diminished the reduction in RV with recumbency by only 0.10 +/- 0.02 L. In contrast, the postural dependence of RV was abolished when the abdomen was tightly supported by elastic straps and maintained constant in configuration during postural changes.(ABSTRACT TRUNCATED AT 250 WORDS)