The mechanical behavior of the lung and chest wall has not been determined in preschool children. We therefore obtained static expiratory pressure-volume (P-V) curves of the respiratory system, partitioned into lung and chest wall components using esophageal (Pes) and airway pressure (Paw) registration in 17 anesthetized children (0.2 to 15.5 yr) in the supine and lateral position. From the P-V curves the inspiratory capacity (IC), the chest wall elastance (Ecw), and the maximal compliance of the respiratory system (Crs) and lungs (C(lung)) were calculated and related to growth. At IC (Paw = 30 cm H(2)O), Pes was the same in the two positions: 11 +/- 3 cm H(2)O. In contrast, at end-expiration (Paw = 0), Pes was close to zero in the lateral position, but markedly positive in the supine position (7 +/- 2 cm H(2)O). C(lung) was similar in both positions and increased with growth. Thus, C(lung) in the lateral position (ml/cm H(2)O) = 0.0017 x length(2.26) (cm), r(2) = 0.90. Crs and IC were approximately 20% greater (p </= 0.001) in the supine position than in the lateral, and correlated strongly (r(2) >/= 0.93) with power functions of length in both positions. Ecw expressed as a fraction of total respiratory system elastance (Ecw/Ers) was 33 +/- 12% in the lateral position and 12 +/- 16% supine (p < 0.001). We conclude that the respiratory mechanics in children correlated closely with body size and showed important differences between the supine and lateral positions.