Background: Sarcopenia, the condition of low muscle mass, is a prognostic factor across many cancer types including bladder cancer. Moreover, sarcopenia was shown to predict severe toxicity induced by many anticancer agents. Conversely, NAC may increase the prevalence of sarcopenia before surgery. This retrospective study aimed to investigate the association of sarcopenia with pathologic response and toxicity in localized bladder cancer patients treated by NAC and RC. Methods: Among the 52 patients who underwent RC after NAC between Jan 2010 and Dec 2016, the preoperative sarcopenia status could be determined for 39 patients. Skeletal muscle cross-sectional area at the third lumbar vertebra was measured on CT scan images, and sarcopenia was defined as skeletal muscle index of < 43 cm2/m2 for men with body mass index (BMI) < 25 kg/m2, SMI < 53 cm2/m2 for men with BMI > 25 kg/m2, and SMI < 41 cm2/m2 for women. The criteria for efficacy was pathologic response defined by pathological down-staging to ≤ pT1N0. Patients were compared using χ2 or Fischer’s exact test when appropriate. Results: 48% of patients were sarcopenic before RC. A significant higher rate of pathological response was found in non-sarcopenic patients compared to sarcopenic patients (13/20 vs 5/19, P = 0.01). More severe chemotherapy related toxicity was found in sarcopenic patients compared to non-sarcopenic patients for neutropenia (P = 0.06), thrombopenia (P = 0.15), and neuropathy (P = 0.23). Among the 21 patients for whom sarcopenia status before chemotherapy was available, 3 patients became sarcopenic and 4 patients initially sarcopenic gained muscle mass on post-chemotherapy CT scan. Conclusions: Sarcopenia is frequent in localized bladder cancer patients. Muscle mass correlates with pathological response to NAC.