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In an attempt to investigate the relationship between respiratory mechanics and pulmonary hemodynamics, we evaluated pulmonary function in 31 infants with left-to-right shunts and subsequent high pulmonary blood flow, undergoing cardiac surgery. Measurements were performed 1 d before and repeated 10 d and 4–5 wk after correction. The age of the patients ranged from 4 d to 24 mo, body weight from 2.7 to 11.8 kg. Pulmonary artery-pressure, assessed by Doppler echocardiography, was preoperatively elevated in 23 patients (group 1), whereas it was within normal values in eight infants (group 2). Respiratory mechanics were measured using the single-breath occlusion technique in sedated infants. To evaluate specific compliance, functional residual capacity was determined by using an open circuit nitrogen washout technique. A reduced preoperative compliance value (mean with 95 confidence interval) was found in group 1: 34.8 (26.5–43.1) mL. kPa−1. After hemodynamic correction, a progressive significant (p< 0.01) improvement was demonstrated at 10 d and 1 mo with values of 47.5 (39.2–55.8) mL. kPa−1and 56.5 (45.6–67.4) mL. kPa−1, respectively. A similar trend was noted evaluating specific compliance with values of 0.27 (0.24–0.30) kPa−1and 0.44 (0.42–0.46) kPa−1, respectively before and after surgery. Preoperative functional residual capacity value was 130 (100–160) mL. In group 2, normal preoperative compliance values were obtained, without significant changes after surgery. In both groups, resistance was within the normal range both before and after surgical correction, and functional residual capacity did not change either. No correlations were found between compliance and pulmonary artery pressure and pulmonary blood flow values. In conclusion, these results show that infants with left-to-right shunts and pulmonary hypertension present with reduced compliance of the respiratory system that rapidly improves after cardiac surgery.