Accurate diagnosis and staging is crucial to ensure uniform allocation to the optimal treatment modalities for non-small cell lung cancer (NSCLC), but may differ between multidisciplinary tumor boards (MDT). Discordance between clinical and pathological TNM-stage is particularly important for patients with locally advanced NSCLC (stage IIIA) because it may influence their chance of allocation to curative intent treatment. We therefore aimed to study agreement on staging and treatment to gain insight in MDT decision making.What is the level of agreement on clinical staging and treatment recommendations between MDT's in stage IIIA NSCLC patients?Eleven MDT's each evaluated the same 10 pathological stage IIIA NSCLC patients in their weekly meeting (n=110). Cases were purposively selected for their challenging nature. All MDT's received exactly the same clinical information and images per patient. We tested agreement in cT-stage, cN-stage, cM-stage (TNM 8th edition) and treatment proposal between MDTs using Randolph's Free-marginal Multirater Kappa.Considerable variation between the MDT's was seen in T-staging (kappa 0.55 [0.34-0.75]), N-staging (kappa 0.59 [0.35-0.83]), overall TNM-staging (kappa 0.53 [0.35-0.72]) and treatment recommendations (kappa 0.44 [0.32-0.56]). Most variation in T-stage was seen in cases with suspicion of invasion of surrounding structures, which influenced treatment recommendations such as (type of) induction therapy. For N-stage, distinction between N1 and N2 disease was an important source of discordance between MDTs. Even regarding M-stage there was variation in two cases. A wide range of additional diagnostics was proposed by the MDT's.This study demonstrates a high variation in staging and treatment of patients with stage IIIA NSCLC between MDT's in different hospitals. Although some variation may be unavoidable in these challenging cases, we should strive for more uniformity.