PURPOSE: To assess the effect of beam hardening on arterial enhancement in thoracoabdominal computed tomographic (CT) angiography in various body sizes in a phantom and in a clinical study. MATERIALS AND METHODS: An abdominal aortic phantom was placed in three cylindrical water containers simulating a small, an intermediate, and a large patient (diameters of 22, 30, and 40 cm, respectively). CT scanning was performed at 80, 100, and 120 kVp, and aortic attenuation was assessed. In a HIPAA-compliant institutional review board-approved study with waiver of informed consent, thoracoabdominal aortic attenuation was assessed in 100 consecutive patients (75 men, 25 women; mean body weight (BW), 79 kg) undergoing thoracoabdominal 64-section CT angiography at 80 kVp. Overall aortic attenuation in patients grouped according to BW (small patients [n = 26], <70 kg; intermediate patients [n = 41], 70-85 kg; large patients [n = 33], >85 kg) was compared. Overall aortic attenuation was correlated with BW, height, body mass index (BMI), body surface area (BSA), and lateral diameter by using linear regression. RESULTS: Comparison of the intermediate- and large-phantom groups relative to the small-phantom group indicated that mean aortic attenuation decreased by 9.7% and 13.4% at 80 kVp, 9.8% and 15.1% at 100 kVp, and 13.8% and 23.8% at 120 kVp, respectively (P < .001). In the clinical study, the overall aortic attenuation decreased by 11.3% and 20.0% in the intermediate- and large-patient groups relative to the small-patient group, respectively (P < .05). Overall aortic attenuation correlated well with BSA (R = -0.454), height (R = -0.421), and BW (R = -0.414), followed by lateral diameter (R = -0.291) and BMI (R = -0.211). CONCLUSION: Beam hardening may substantially contribute to reduced arterial enhancement in larger patients undergoing thoracoabdominal CT angiography. The beam-hardening effect on arterial enhancement is more pronounced at higher tube voltages.