Purpose <p>Low muscle mass. and skeletal muscle loss have been associated with prognosis in patients with cancer. This study evaluated the effects of changes in the skeletal muscle index (SMI) on overall survival in patients with limited-stage small-cell lung cancer (LD-SCLC) undergoing concurrent chemoradiotherapy (CCRT).</p> Methods <p>This retrospective study included 55 patients with LD-SCLC who underwent CCRT. Body composition analysis was performed at baseline and post-treatment to determine changes in body mass index (BMI), the SMI at the fourth thoracic vertebral level (T4), and the SMI and areas of fat at the third lumbar vertebral level (L3). The prognostic impact of these parameters was evaluated.</p> Results <p>At baseline, 44.6% of patients had L3 low muscle mass. according to Korean population-specific criteria. Median changes in the L3 SMI, T4 SMI, BMI, and body weight after CCRT were − 5.8 cm²/m² (<i>P</i> &lt; 0.001), 0.1 cm²/m² (<i>P</i> = 0.613), -0.2&#xa0;kg/m² (<i>P</i> = 0.316), and − 0.7&#xa0;kg (<i>P</i> = 0.228), respectively. Univariate analysis associated Eastern Cooperative Oncology Group (ECOG) performance status 2 (<i>P</i> &lt; 0.001), baseline T4 low muscle mass, defined as the lowest T4 SMI quartile (<i>P</i> = 0.016), and a decrease in the L3 SMI during treatment (<i>P</i> = 0.011) with an increased mortality risk. Multivariate analysis identified ECOG 2 (hazard ratio = 5.815; <i>P</i> = 0.003) and a decrease in the L3 SMI during treatment (hazard ratio = 2.172; <i>P</i> = 0.027) as risk factors for poor prognosis.</p> Conclusion <p>Skeletal muscle loss during CCRT, particularly a decrease in L3 SMI, was associated with inferior overall survival in LD-SCLC patients. Monitoring and mitigating skeletal muscle loss may improve patient prognosis.</p>