A comparative study of the diagnosis of intracardiac left-to-right shunts by isotope radiography on the one hand and oxymetry, abnormal catheter trajectory and angiocardiography on the other, was performed on a series of 110 patients, comprising 12 normals, 46 valvulopathies or cardiomyopathies without shunts, 23 ASDs, 22 VSDs, and 7 other shunts. Intravenous injection of Technetium 99 m with scintigraphic imagery and interpretation of time-activity curves detected even small left-to-right shunts and appeared to be more sensitive than oxumetry. The estimation of the size of the shunt correlated well with oxymetry for VSDs but not for ASDs. The localisation of the shunt was more difficult. The atrial level of the defect was detected in some but not the majority of cases. The isotopic technique appeared unreliable in the presence of severe haemodynamic disturbances. The isotopic method is a quick means of detecting left-to-right shunts without measurable risk, particularly useful in the investigation of children and young adults thought to have left-to-right shunts.