The noninvasive nature of the radionuclide angiocardiography provided a useful approach for the evaluation of left-to-right cardiac shunts (LRCS). While the qualitative information can be obtained by inspection of serial radionuclide angiocardiograms, the quantitative information of radionuclide angiocardiography can be obtained by the analysis of time-activity curves using advanced computer system. The count ratios metod and pulmonary-to-systemic flow ratio (QP/QS) by gamma variate fit rnethod were used to evaluate the accuracy of detection and localization of LRCS. One hundred and ten time-activity curves were analyzed. There were 46 LRCS (atrial septai defects 11, ventricular septal defects 22, patent ductus arteriosus 13) and 64 normal subjects. By compute:r analysis of time-activity curves of the right atriurn, ventricle and the lungs separately, the count ratios modiiied by adding the rnean cardiac transit time were calcuiated in each anatomic site. In normal subjects the mean count ratios in the right atrium, ventricle and lungs were 0.24 on average. In atrial septal defects, the count ratios were high in the right atrium, ventricle and lungs, whereas in ventricular septal defects the count ratios were higher only in the right ventricle and lungs. Pateot ductus arteriosus showed normal count ratios in the heart but high count ratios were obtained in the 1ungs. Thus, this count ratios method could be separated norma! from those with intracardiac or extracardiac shunts, and moreover, with this method the localization of the shunt level was possible in LRCS. Another method that could differentiate the intracardiac shunts from extracardiac shunts was measuring QP/QS in the left and right lungs. In patent ductus arteriosus, the left lung QP/QS was higher than those of the right lung, whereas in atrial septa! defects and ventricular septal defects QP/ QS ratios were equal in both lungs. From this study, it was found that by rneasuring QP/QS separately in the lungs, intracardiac shunt could be differenciated from ext.racardiac shunts.