Anaplastic thyroid cancer is the most aggressive form of cancer found in humans. Usually it is0aeasily diagnosed; however at times other diseases are mistaken for anaplastic thyroid cancer. We0apresent a case of primary lung adenocarcinoma presenting with features that appeared as anaplastic0athyroid cancer. The 43-year-old female patient was diagnosed with anaplastic thyroid cancer at a0alocal clinic just before presenting to our hospital. At the clinic she had a neck node excisional biopsy0aand was informed of the diagnosis of anaplastic cancer. On admission to our hospital, very large0abilateral thyroid masses, and lymphadenopathy involving multiple cervical lymph nodes was0aobserved; therefore, we started chemoradiotherapy. The patient showed dramatic improvement and0awe began to think of other potential etiologies. A FDG-PET study showed increased uptake at the0aleft lower lung area corresponding to a pneumonic consolidation; a TBLB was performed, and0areported as poorly differentiated adenocarcinoma. We referred the patient to the oncology department.0aThe patient died after two cycles of systemic chemotherapy