PURPOSE: To determine the predictability of an aberrant hepatic artery by detection of a vessel in the portocaval space or fissure for the ligamentum venosum, as seen on arterial-phase spiral CT images. MATERIALS AND METHODS: Axial spiral CT scans (10mm section thickness, 10mm table feed) were obtained in 100 patients with hepatic mass and were examined by two radiologists. In each case, each determined whether a vessel was located in the portocaval space or fissure for the ligamentum venosum, and the type of aberrant artery. All patients underwent conventional angiography and the results were interpreted by another radiologist and compared with the results as shown on CT. RESULTS: Twelve-one cases with a vessel within the portocaval space and 14 with a vessel within the fissure for the ligamentum venosum showed variation of the hepatic artery. When a vessel was located transversely in the portocaval space, sensitivity, specificity, and positive and negative predictive value of an aberrant hepatic artery were 94%, 100%, 100%, and 99%, respectively; when a vessel was located in the fissure for the ligamentum venosum, the corresponding rates were 88%, 100%, 100%, and 98%. CONCLUSION: The detection on arterial-phase spiral CT images of a vessel in the portocaval space or fissure for the ligamentum venosum can reliably predict the existence of an aberrant hepatic artery.