Background: In patients with cancer, sarcopenia is associated with treatment related complications, treatment cessation, poor quality of life and reduced overall survival. Despite this, there is limited knowledge about changes in skeletal muscle mass during chemotherapy. The aim of this systematic review and meta-analysis was to investigate the change of skeletal muscle mass and sarcopenia during chemotherapy treatment among patients with lung cancer. Methods: A systematic literature search was conducted in three databases, PubMed, EMBASE and Web of Science. Observational studies with patients with lung cancer were eligible for inclusion if skeletal muscle mass was measured before and after receiving chemotherapy treatment. Results: Ten cohort studies with a total of 867 participants met the inclusion criteria. During 5.2 ± 2.9 months of chemotherapy treatment, patients with lung cancer experienced a significant loss of skeletal muscle mass with a standardized mean difference (SMD) of: -0.25 (95% CI -0.47 to -0.03). The pretreatment prevalence of sarcopenia varied across studies from 35% to 74%. Only one study reported prevalence of sarcopenia both before and after chemotherapy treatment with an increase from 35% to 59%. Conclusion: The present data demonstrate a marked loss of skeletal muscle mass in patients with lung cancer undergoing chemotherapy treatment, as well as a high prevalence of sarcopenia. As sarcopenia is associated with poor clinical outcomes, it seems important to include and use assessments of skeletal muscle mass in clinical practice to identify patients in need for interventions. Moreover, interventional studies to hinder development of sarcopenia are needed.