PURPOSE: To construct a useful index for use as a prognostic factor in cases of aortic aneurysm. MATERIALS AND METHODS: Using CT or EBT, we studied nine ruptured aortic aneurysms, 40 unruptured aneurysms, and 42 normalaortas, measuring aortic diameter and wall thickness. Systolic, mean or diastolic blood pressure was used as apressure parameter. Tangential stress(TS) and the tangential stress index ( TSI) were calculated by modified Laplace's law. RESULTS: Average diastolic TS's (TSI's) were 1938 (4.13), 905 (1.84) and 554 (0.94) in rupturedaneurysm, unruptured aneurysm and normal groups, respectively (p<0.01). ROC curves of diastolic TS and TSI wereseen in a "useful study" zone. With a threshold of 1230 (2.90) for TS (TSI), the sensitivity and thespecificity for differentiation of ruptured and unruptured aneurysms were 100% (100%) and 75% (88%), and thepositive and the negative predictive values were 47% (64%) and 100% (100%), respectively (p<0.01). Amongsystolic, mean and diastolic TS 's and TS I 's, the diastolic TSI showed the highest specificity at its maximalsensitivity. CONCLUSION: Diastolic TSI is a more accurate prognostic factor for aortic aneurysm.