BACKGROUND: A decrease in skeletal muscle mass and function, defined as sarcopenia, is associated with poor postoperative outcome in patients with cancers. Although systemic or local immune status impacts cancer progression, the relationship between sarcopenia and these statuses remains unclear. The aim of this study is to investigate the clinical impact of sarcopenia and its relationship to immune systems in patients with extrahepatic cholangiocarcinoma (ECC). METHODS: A total of 110 consecutive ECC patients with curative resection between 2005 and 2014 were enrolled. Sarcopenia was determined from skeletal muscle index, assessed by a L3 skeletal muscle mass on axial computed tomography images, and their relationships with patients' clinicopathological characteristics and survival were evaluated. Systemic immune status was calculated using preoperative laboratory data, and tumor-infiltrating (TI) immune cells (CD8+ T cells, CD66b+ neutrophils, CD163+ M2 macrophages) assayed by immunohistochemistry, and their relationship to sarcopenia were evaluated. RESULTS: Sarcopenia was present in 31 patients (28.2%). Patients with sarcopenia had a worse recurrence-free survival (HR 1.87, p = 0.009) and overall survival (OS) (HR 2.47, p = 0.0004) than patients without sarcopenia. Moreover, patients with sarcopenia had a higher level of platelet-lymphocyte ratio (159 vs. 119; p = 0.003) and lower number of TI CD8+ T cells (47 vs. 66 cells/spot; p = 0.03) than patients without sarcopenia. On multivariate analysis, the presence of sarcopenia (HR 2.60, p = 0.0008) was an independent predictor of poor OS. CONCLUSIONS: Our data showed that sarcopenia and systemic or local immune cells may interact with each other and play a pivotal role in clinical outcomes of patients with ECC.