e14061 Background: Severe skeletal muscle loss (sarcopenia) is a principle property of cancer cachexia and is found to be a hallmark of poor prognosis in patients with advanced non-small cell lung cancer (NSCLC). With the latest advancements of PD-1 and PD-L1 checkpoint immunotherapy, we aim to examine changes in sarcopenia before and after treatment with these agents. Methods: : In this study, patients with stage IIIB/IV NSCLC receiving PD-1 and PD-L1 immune checkpoint inhibitors were evaluated. CT images obtained before and after treatment were used for skeletal muscle analyses with the SliceOmatic software (Tomovision) at the level of the first lumbar vertebra. Skeletal muscle index (SMI) was assessed by measuring the cross-sectional muscle area, normalized to patient height. Height, weight, disease progression status, and overall survival (OS) was extracted from EPIC under an IRB-approved protocol. Data was then compared with baseline and clinical outcome was used for survival analysis. Results: In 100 pre-treatment subjects (48% women, mean age of 68), patients with < 32 cm2/m2 SMI had a significantly lower OS (median OS = 1.41 mo, n = 16) compared to those with SMI > 32 cm2/m2 (median OS = 9.44 mo, n = 84, p = 0.024). In 74 patients with pre- and post-treatment data, (avg 2.66 mo interval) an increase of > 5% SMI from baseline occurred in 17 (23%) patients, while a decrease of > 5% SMI from baseline occurred in 26 (35%) patients. Mean reduction in SMI from pre- to post-immunotherapy was 0.921 cm2/m2 while median reduction in SMI was 1.087 cm2/m2. Conclusions: Patients with high pretreatment SMI had a significantly greater overall survival when compared to those with low pretreatment SMI. Two-thirds of patients experienced stability or increase in SMI during immunotherapy. These results suggest that immune checkpoint inhibitors may dampen mechanisms of cancer cachexia in some patients.