Purpose: To test whether quantitative morphologic evaluation of the atrial appendages using multi-slice spiral CT was feasible to determine the atrial situs. Materials and Methods: 176 consecutive 16-slice CT examinations in 148 patients (median age, 1 year; range, 1 day – 56 years) with congenital heart disease were enrolled in this study. Atrial situs is solitus in 160 CT examinations, inversus with a congenitally corrected transposition of the great arteries in 1, right isomerism in 11, left isomerism in 1, and ambiguous in 3. Quantitative evaluation was performed along the long axis of the atrial appendages. Atrial appendage index (AAI) was given by dividing a diameter from opening to tip of the appendage by a diameter of appendage opening. Left-to-right index (LRI) of atrial appendages was given by dividing left AAI by right AAI. In atrial situs solitus, left AAIs were compared with right AAIs. AAI and LRI were compared between situs solitus and right isomerism. Atrial situs determined by the morphology of the atrial appendages was correlated with atrial situs determined by the relationship between the upper lobar bronchus and the pulmonary artery. Results: In atrial situs solitus, left AAIs (3.2짹1.0) were significantly larger than right AAIs (1.2짹0.3) (P < 0.001). Left (1.2짹0.2) and right (1.1짹0.2) AAIs in right isomerism were significantly smaller than left AAIs in situs solitus (P < 0.001) but were not significantly different from right AAIs in situs solitus (P = 0.65, 0.3). LRIs (2.8짹1.1) in situs solitus were significantly larger than those in right isomerism (1.1짹0.2)(P < 0.001). Atrial situs determined by the morphology of the atrial appendages was discordant with that determined by the relationship between the upper lobar bronchus and the pulmonary artery in 1.4% (2/148). Regarding associated cardiac malformations and visceral situs, atrial situs determined by the morphology of the atrial appendages was accurate in two discordant patients. Conclusion: Quantitative morphologic evaluation of the atrial appendages is feasible to determine the atrial situs, and is particularly helpful in situs ambiguous patients.