OBJECTIVE To prospectively evaluate a dual-phase helical acquisition in computed tomography (CT) of the liver. METHODS Two helical CT acquisitions were performed in 103 patients: hepatic arterial phase (HAP) 15 seconds after injection of 2 mL/kg contrast material at a rate of 6 mL/sec and the portal venous phase (PVP) 90 seconds after injection. Detection of focal liver lesions, vascular anatomic features, and perfusion abnormalities were assessed by two reviewers. Liver enhancement was also calculated on every section. RESULTS Of 119 detected focal liver lesions, nine were seen during HAP only and 40 during PVP only. According to a 0-3 score, visualization of the arterial anatomic landmarks was better during HAP (P < .0001). Five right replaced and two left replaced hepatic arteries were visualized only during HAP. Parenchymal perfusion abnormalities were found in 32 patients during HAP and in four patients during PVP. CONCLUSIONS A dual-phase acquisition depicted 8% additional focal liver lesions and outlined the entire vasculature of the liver.