Lung cancer (LC) has a high rate of anorexia, which negatively affects quality-of-life and prognosis; however prevalence values may vary as per diagnostic test. There is no standard for anorexia diagnosis, currently the anorexia cachexia scale (A/CS) has been proposed as a tool for diagnosing anorexia with a consensus cutoff value of ≤24, nonetheless a validated cutoff value is required. The A/CS was evaluated in advanced Non-Small Cell Lung Cancer (NSCLC) patients to establish a cutoff value. The appetite item from the QLQ-C30 questionnaire and survival served as a standard reference. The cutoff value was associated with clinical and nutritional characteristics along with quality-of-life. Three hundred and twelve (312) NSCLC patients were evaluated. The mean A/CS value was 31 ± 9 and the identified cutoff value was 32.5 (sensitivity: 80.3% and specificity: 85%). The proportion of anorexia accurately diagnosed with the cutoff value of 24 was 26%, while with 32 it was 50%. The A/CS cutoff value of 32 was associated with clinical parameters, nutritional consumption, and quality-of-life, and independently associated with overall survival. A score of ≤32 in the A/CS is proposed for anorexia diagnosis in order to identify patients at risk of complications involving malnutrition related to LC.