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OBJECTIVE The aim of the study was to clarify the association of skeletal muscle mass and the prognosis of unresectable pancreatic ductal adenocarcinoma (PDAC) treated with gemcitabine plus nab-paclitaxel (GnP). METHODS We included 124 unresectable PDAC patients who received GnP chemotherapy. Skeletal muscle mass of the third lumbar vertebrae (L3) level was measured by computed tomography immediately before GnP initiation, and the skeletal muscle index (L3-SMI) was calculated. Sarcopenia was defined as L3-SMI less than 42 cm2/m2 in male patients and less than 38 cm2/m2 in female patients. RESULTS Sarcopenia was found in 63 patients (50.8%). There was no significant difference in overall survival (OS) between sarcopenia and nonsarcopenia patients; however, in elderly patients (>70 years), the OS of sarcopenia patients was significantly poorer than that of nonsarcopenia patients (390 vs 631 days, respectively; hazard ratio, 2.64; 95% confidence interval, 1.33-5.23). Multivariate analyses in elderly patients revealed that sarcopenia and tumor stage were independent poor prognostic factors. Despite the short OS of elderly sarcopenia patients, there were no significant differences in progression-free survival or response rate. CONCLUSIONS Sarcopenia diagnosed by L3-SMI is a prognostic factor in elderly patients who receive GnP for unresectable PDAC. However, GnP exhibits a certain efficacy in sarcopenia and nonsarcopenia patients.