OBJECTIVE Helical CT scanners now allow sequential arterial phase and portal venous phase scans of the entire liver to be obtained during a single bolus injection of contrast material. The purpose of this study was to determine if arterial phase scans improve detection of small (< or = 1.5 cm) malignant hepatic neoplasms when compared with portal venous scans alone. METHODS Dual-phase helical CT of the liver was done in 96 patients referred for known or suspected malignant hepatic lesions. Malignant hepatic neoplasms were detected in 38 patients (27 with at least one small neoplasm), one patient had undetected metastases, one patient had a benign hepatic neoplasm, and 56 patients had no hepatic neoplasm. Proof of individual neoplasms was based on biopsy results, surgical findings, or findings on other imaging studies (primarily follow-up CT). The absence of disease was established by surgical or autopsy findings, findings on subsequent imaging studies, or a combination of clinical and laboratory data. A total of 150 ml of 60% nonionic contrast material was infused at 5 ml/sec followed by sequential arterial phase and portal venous phase helical scans of the liver. Three radiologists retrospectively reviewed the scans. Individual lesions were measured and the conspicuity of each lesion on arterial phase and portal venous phase scans was compared. The percentage of patients in whom some malignant neoplasms were detected better on the arterial phase scan was calculated using categories based on lesion size and typical tumor vascularity. RESULTS In 10 (37%) of 27 patients who had at least one small malignant neoplasm, lesions 1.5 cm or less in diameter were only visible or were more conspicuous on the arterial phase scan. No malignant neoplasms more than 1.5 cm in diameter were visible only on the arterial phase scan. In four (11%) of 38 cases, malignant neoplasms more than 1.5 cm in diameter were more conspicuous on the arterial phase scan. The arterial phase scans improved lesion conspicuity in nine (39%) of 23 patients who had typically hypervascular neoplasms, whereas lesion conspicuity was improved in three (20%) of 15 patients who had typically hypovascular neoplasms (p = .02). The arterial phase scan resulted in the false-positive detection of lesions in two (2%) of 96 cases. CONCLUSIONS Arterial phase helical CT of the liver improves detection of some small, malignant hepatic neoplasms when performed in addition to portal venous scanning. The value is greatest in those patients who have hypervascular neoplasms.