Background: Accurate diagnosis and staging are crucial to ensure uniform allocation to the optimal treatment methods for non-small cell lung cancer (NSCLC) patients, but may differ among multidisciplinary tumor boards (MDTs). Discordance between clinical and pathologic 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 into MDT decision-making. Research Question: What is the level of agreement on clinical staging and treatment recommendations among MDTs in stage IIIA NSCLC patients? Study Design and Methods: Eleven MDTs each evaluated the same 10 pathologic stage IIIA NSCLC patients in their weekly meeting (n = 110). Patients were selected purposively for their challenging nature. All MDTs 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 among MDTs using Randolph's free-marginal multirater kappa. Results: Considerable variation among the MDTs was seen in T staging (κ, 0.55 [95% CI, 0.34-0.75]), N staging (κ, 0.59 [95% CI, 0.35-0.83]), overall TNM staging (κ, 0.53 [95% CI, 0.35-0.72]), and treatment recommendations (κ, 0.44 [95% CI, 0.32-0.56]). Most variation in T stage was seen in patients with suspicion of invasion of surrounding structures, which influenced such treatment recommendations as induction therapy and type. For N stage, distinction between N1 and N2 disease was an important source of discordance among MDTs. Variation occurred between 2 patients even regarding M stage. A wide range of additional diagnostics was proposed by the MDTs. Interpretation: This study demonstrated high variation in staging and treatment of patients with stage IIIA NSCLC among MDTs in different hospitals. Although some variation may be unavoidable in these challenging patients, we should strive for more uniformity.@en