Objective <p>Mass-forming intrahepatic cholangiocarcinomas (MF-ICCs) can be classified into ductal and parenchymal types using magnetic resonance imaging (MRI). We aimed to subclassify MF-ICC into biliary and parenchymal types based on ultrasound (US) findings and to investigate the differences in their contrast-enhanced ultrasound (CEUS) patterns, clinicopathologic features, and prognosis.</p> Methods <p>In this study, 141 patients who underwent US with pathologically proven MF-ICC from two hospitals were retrospectively enrolled. MF-ICCs were divided into biliary (bMF-ICCs) and parenchymal MF-ICC (pMF-ICCs) based on the signs of bile duct dilation in US images. Clinicopathological, imaging, and short-term survival data were collected from medical records and compared.</p> Results <p>Among 141 patients (61.96 ± 10.15 years, 83 men), bMF-ICCs (33/141, 23.4%) showed significantly more CEA ≥ 5 µg/L (42.4% vs 20.2%, <i>p</i> = 0.01), microvascular invasion (54.5% vs 10.2%, <i>p</i> &lt; 0.001), lymph node metastasis (48.5% vs 5.6%, <i>p</i> &lt; 0.001), bile duct invasion (48.5% vs 5.6%, <i>p</i> &lt; 0.001), and high Ki-67 expression (63.6% vs 38.9%, <i>p</i> = 0.01) than pMF-ICCs. Pathologically, bMF-ICCs were more inclined toward the large duct type (78.1% vs 11.7%, <i>p</i> &lt; 0.001). In addition, the bMF-ICCs were usually located in the left lobe of the liver (63.6% vs 41.7%, <i>p</i> = 0.03). pMF-ICCs showed better overall survival than bMF-ICCs (<i>p</i> = 0.04).</p> Conclusions <p>Subclassification of MF-ICCs into biliary and parenchymal types based on US is useful for discriminating clinicopathological characteristics.</p> Critical relevance statement <p>The subclassification of mass-forming intrahepatic cholangiocarcinoma (MF-ICC) into biliary (bMF-ICC) and parenchymal (pMF-ICC) subtypes using ultrasound can provide clinicopathological and prognostic information before surgery.</p> Key Points <p><UnorderedList Mark="Bullet"> <ItemContent> <p>We subclassified mass-forming intrahepatic cholangiocarcinomas into biliary and parenchymal types using ultrasound.</p> </ItemContent> <ItemContent> <p>Biliary and parenchymal types have different clinicopathological features and postsurgical outcomes.</p> </ItemContent> <ItemContent> <p>Biliary type above and below 50 mm exhibits different unfavorable clinicopathological characteristics.</p> </ItemContent> <ItemContent> <p>Our classification has certain similarities with MRI classification in clinicopathological characteristics.</p> </ItemContent> </UnorderedList></p> Graphical Abstract <p></p>