OBJECTIVE Contrast-enhanced helical CT images of the liver are usually obtained during the portal vein phase (PVP), during which the parenchyma achieves peak enhancement. The purpose of this study was to determine whether the addition of arterial-phase (AP) scans would lead to improved characterization of focal lesions. METHODS We reviewed the CT appearance of 102 focal lesions with a proven diagnosis. In the first part of the study, we assessed whether the addition of AP scans influenced the diagnostic performance of the three observers. In the second part of the study, we analyzed the morphologic appearance revealed on CT scans of the different types of lesions in the AP and PVP. RESULTS The addition of AP scans led to a significant increase in the number of correct diagnoses: 71 lesions (70%) were correctly diagnosed with CT scans in both the AP and the PVP, compared with 54 lesions (53%) correctly diagnosed with CT scans in the PVP alone (p < .01). The largest difference was observed in the diagnosis of focal nodular hyperplasia (FNH) and adenoma (16/24 correct diagnoses instead of 6/24) (p < .005). Conversely, AP images did not significantly contribute to the diagnosis of hemangiomas and metastases. The following morphologic features were seen much more often on AP scans than on PVP scans and had a high positive predictive value (PPV): spoke-wheel pattern (FNH; PPV, 100%), central feeding vessel (FNH; PPV, 100%), and heterogeneous appearance with hyperdense components (hepatocellular carcinoma; PPV, 75%). CONCLUSIONS Our data show that the radiologists' evaluation of CT scans in both the AP and the PVP improves the differentiation of hepatocellular carcinoma and FNH from other types of hepatic neoplasms.