The purpose of this study was to determine the effect of chestwall distortion (CWD) on the measurement of pulmonary compliance (CL) and resistance (RL). Inductance plethysmography was used in 15 preterm infants to determine CWD as total compartmental displacement ratio (TCDR) and out of phase movement between ribcage and abdomen as phase shift (PS). Flow was measured by pneumotachography and esophageal pressure change (Pe) with a water-filled catheter. CL and RL were calculated by linear regression analysis. Seven infants (mean +/- SD: BW, 1,484 +/- 186 g, GA 32.4 +/- 2.2 weeks, age 8.7 +/- 4.7 days) had a breathing pattern characterized by episodes with a high degree of CWD, followed by periods with minimal CWD (distortion group). In this group lung function measurements were analyzed separately during periods with and without CWD. The remaining 8 infants (BW, 1,244 +/- 233 g, GA 30.4 +/- 2.4 weeks, age 7.4 +/- 3.1 days) always breathed with minimal CWD, and the measurements in this group (non-distortion group) were used as a reference for the values obtained in the distortion group. Measurements of TCDR, PS, CL, RL, and tidal volume (VT) obtained in the absence of CWD were not significantly different between distortion and non-distortion groups. The measurements obtained in the presence of CWD showed a significantly higher TCDR and PS, but CL and RL were not significantly different from the CL and RL measurements obtained in the distortion and non-distortion groups in the absence of CWD. The only significant effect of CWD was a reduction in VT.(ABSTRACT TRUNCATED AT 250 WORDS)