BACKGROUND: Preoperative identification of systemic and pulmonary venous return is essential for surgical design in situs ambiguus. This study was carried out to evaluate anatomy of systemic and pulmonary venous return and to assess clinical efficacy of magnetic resonance imagine(MR) by comparing with results of cardiac catheterization(Cath) and echocardiography(Echo). MATERIALS AND METHODS: MR performed on 22 patients with cardiac situs ambiguus(right isomerism ; 13, left isomerism ; 9). MR findings were compared with the findings of Cath and Echo for the assessment of diagnostic accuracy of MR in 19 patients. RESULTS: 1) Interruptions of IVC with azygous continuation were found in all patients of left isomerism. But IVC was drained to right of left sided atria in right isomerism. 2) Brlateral SVC were found in 12 of 22 situs ambiguus(left isomersm ; 5, right isomerism ; 8). 3) Total anomalous pulmonary venous returns(TAPVR) were found in 7 of 12 right isomerism. Location of vertical veins were as follows ; prearterial(n=1), retroarterial-prebronchial(n=3), retrobronchial(n=3). In 5 patients of remained 6 right isomerism, pulmonary venous returns(PAPVR) in which right and left pulmonary veins entered to right and left atrium respectively, were found in 5 of 9 left isomerism. 4) Compared with Cath and Echo findings(n=19) in which MR, Cath and Echo were performed simultaneously, TAPVR were found in 6 cases on MR but 2 cases on Cath and Echo. The cases that were not detected by Cath showed severe decrement of pulmonary flow due to hypoplasia of pulmonary artery or obstruction of pulmonary vein. On MR, accurate anatomy of PAPVR were found only in 4 cases. CONCLUSIONS: MR can provide accurate and complete imaging of systemic and pulmonary venous return in sitrs ambiguus. Especially, MR is superior to Cath or Echo in depiction of TAPVR with severe decrement of pulmonary flow of obstruction of pulmonary vein, PAPVR and bilateral SVC.