Since 1973, the balloon catheter test with a Swan-Ganz, Dotter-Lukas or Fogarty catheter has been attempted in our department in cases of atrial septal defect (ASD) and patent ductus arteriosus (PDA) during cardiac catheterization and operation. Accurate informations concerning size, number and location of ASD were obtained during cardiac catheterization studies by using the balloon catheter. The difference between the size of the defect measured at catheterization and at operation was less than 1 mm in diameter. Size of the defect ranged from 13 to 44 mm in diameter (25.7 mm on the average) and bacame larger with patient's age. Measurement of left-to-right shunt ratios before and after the temporary balloon occlusion of the defect was of help in making a preoperative diagnosis of ASD with multiple defects. Size of foramen ovale ranged from 3 to 13 mm in diameter (5.2 mm on the average) and was not related to age. Size of the ductus in cases with PDA was measured during cardiac catheterization by using the balloon catheter. The difference between the size of the ductus measured with the balloon technique at catheterization and with a measuring rule at operation was less than 4 mm in diameter. The difference revealed the extensibility of the ductus. The temporary balloon occlusion of the ductus aided in determining presence of other cardiac anomalies. In cases of isolated PDA, the murmur became inaudible with the occlusion of the ductus. A balloon catheter inflated with radiopaque dye was used for measurements at cardiac catheterization and at operation. Persistent arrhythmias or other adverse effects of this procedure were not observed.