The accuracy of echocardiographic estimation of left-to-right shunts was studied prospectively in children with secundum ASD. Fifty-one consecutive patients were examined from March 1987 to August 1991. Mean age was 64/12 years with a minimum age of 12/12 and a maximum of 131/12 years. Twenty-five children were included in the first part of the study. The ratios of right and left atrial and ventricular areas and pulmonary and aortic diameters were correlated with the left-to-right shunts determined by oximetry (mean shunt 48.7%; min. 10.1%, max. 73.2%). The pulmonary/aortic diameter ratio provided the closest correlation (r = 0.95; p < 0.01). During the second part of the study the left-to-right shunt was estimated using the regression equations developed from part one. Thus, in 26 subsequent patients these values were compared with oximetric shunt values. Again, the pulmonary/aortic diameter ratio provided the closest correlation (r = 0.97). The pair comparison yielded a difference of 0.7 +/- 3.8%. Based on these results since 1991, surgery for ASD II has been performed without preoperative cardiac catheterization in 33 patients, provided echocardiographic shunt estimation was > or = 48%.