A woman in her 60s was referred for evaluation of a rapidly growing thyroid goiter that coincided with the development of dysphagia, orthopnea, and hoarseness. She denied any constitutional symptoms of hypothyroidism or hyperthyroidism and had no history of irradiation. Physical examination revealed a firm, diffusely enlarged right thyroid with tracheal deviation. An immobile right vocal fold was encountered on laryngoscopy. Ultrasonography showed a 9.7-cm soft-tissue neoplasm arising from the right thyroid lobe engulfing the right common carotid artery and multiple enlarged lymph nodes at level IV of the right side of the neck. Fine-needle aspiration cytology (FNAC) of the thyroid mass demonstrated an adenomatoid nodule. Flow cytometry and FNAC of the level IV node did not show any immunophenotypic abnormality. Computed tomographic images ( Figure, A and B) demonstrated a mass in the right lobe of the thyroid with focal calcifications with extension into the parapharyngeal and prevertebral spaces, laryngotracheal deviation, encasement of the right common and internal carotid arteries with obliteration of the internal jugular vein, and enlarged cervical lymph nodes.Given the rapid growth of the mass, imaging findings, and vocal fold paralysis, incisional biopsy of thyroid mass and excisional biopsy of the level IV node were performed. Pathologic evaluation of the thyroid mass and lymph node revealed atypical single-cell infiltrate of sheets of large cells with variably prominent nucleoli, vesicular chromatin, and eosinophilic cytoplasm with destruction of the follicular epithelium (Figure, C) that were positive for CD20 (Figure, D), Bcl-6, and Bcl-2 by immunohistochemical analysis (IHC) but showed no light chain, CD5, or CD10 expression by flow cytometry.