['The apelin-APJ system regulates angiogenesis, and is overexpressed in several types of cancer. The aim of this study was to clarify the role of the apelin-APJ system in the angiogenesis of hepatocellular carcinoma (HCC).', 'Expressions of angiogenic factors and vascular markers were investigated in specimens from 90 HCC patients. A subcutaneous HCC tumor mouse model was treated with the APJ antagonist, F13A, and tumor growth and vascular development were assessed.', 'APJ expression was observed in arteriole-smooth muscle. Higher amounts of APJ(+)-arteriole and apelin were detected in tumors (p<0.001 for both). APJ(+)-arteriole and apelin expression were more commonly observed in moderately- and poorly-differentiated than in well-differentiated HCC (p ≤ 0.003). HCC with irregular dilated arteries expressed higher levels of apelin (p=0.012). Tumor growth was inhibited by treatment with F13A (p<0.001), and arterioles were decreased in the treated group (p=0.047), in vivo.'] Does the apelin-APJ system induce tumor arteriogenesis in hepatocellular carcinoma?