Cachexia and sarcopenia are associated with poor outcome and increased chemotherapy-induced toxicity in lung cancer patients. However, the complex interplay of obesity, sarcopenia and cachexia, and its impact on survival in the context of first-line-chemotherapy is not yet understood. In 200 consecutively recruited lung cancer patients (70 female, mean age 62y; mean BMI 25 kg/m2; median follow-up 15.97 months) with routine staging-CT before and after chemotherapy (CTX, mean interval: 4.3 months), densitometric quantification of total (TFA), visceral (VFA), and subcutaneous-fat-area (SFA), inter-muscular-fat-area (IMFA), muscle-density (MD), muscle-area (MA) and skeletal-muscle-index (SMI) was performed retrospectively to evaluate changes under chemotherapy and the impact on survival. We observed increases in TFA, VFA, SFA, VFA/SFA, and IMFA (p<0.05-0.001), while there were decreases in MA, MD and BMI (p<0.05-0.001) after chemotherapy. High pre-therapeutic VFA/SFA was a predictive factor for poor survival (HR = 1.272; p = 0.008), high pre-therapeutic MD for improved survival (HR = 0.93; p<0.05). Decrease in BMI (HR = 1.303; p<0.001), weight (HR = 1.067; p<0.001) and SMI (HR = 1.063; p<0.001) after chemotherapy were associated with poor survival. Patients with ≥4 CTX-cycles showed increased survival (17.6 vs. 9.1months), less muscle depletion (SMIdifference: p<0.05) and no BMI loss (BMIdifference: p<0.001). After chemotherapy, patients exhibited sarcopenia with decreased muscle and increased adipose tissue compartments, which was not adequately mirrored by BMI and weight loss but by imaging. Particularly sarcopenic patients received less CTX-cycles and had poorer survival. As loss of BMI, weight and muscle were associated with poor survival, early detection (via imaging) and prevention (via physical exercise and nutrition) of sarcopenia may potentially improve outcome and reduce chemotherapy-induced toxicity.