To clarify which hemodynamic measurement correlates best with lung mechanics in infants with congenital heart disease and left-to-right shunts, dynamic pulmonary function tests and echocardiography were performed in 26 infants with such disease (study infants) and in 37 normal, healthy infants (control infants). The tidal volume and pulmonary compliance (CL) were lower and airway resistance higher in infants with congenital heart disease than in control infants. A significant correlation was demonstrated between CL, expiratory resistance (Re), and the right pulmonary artery-to-aortic size ratio (RPA/DAO). CL and Re also correlated well with the corrected acceleration time square root of RR ratio (ACT/square root of RR: ACT, acceleration time and RR: length of the cardiac cycle) of pulmonary flow velocity. Stepwise multiple regression analysis revealed that RPA/DAO correlated best with both CL and Re. It is concluded that infants with congenital heart disease and left-to-right shunts have lower lung compliance and higher expiratory airway resistance than normal children, and that RPA/DAO is the echocardiographic parameter that correlates best with the changes in lung mechanics.