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Metastatic carcinomas to the thyroid are quite rare in daily cytology practice. However, when present they may produce a diagnostic dilemma, particularly when they share some morphologic similarities with primary thyroid lesions and when occurring in patients with occult malignant history. Herein, we report a case of metastatic gastric signet ring cell carcinoma to the thyroid. Our patient presented with an isolated right thyroid nodule, which was clinically considered to be a primary thyroid neoplasm. Fine-needle aspiration (FNA) cytology of the nodule revealed a cellular specimen with cohesive fragments and scattered individual neoplastic cells. The neoplastic cells had enlarged nuclei, fine chromatin, and inconspicuous nucleoli. Nuclear crowding, molding, and grooving were prominent. Intranuclear inclusion-like clearance was identified. Some tumor cells also had eccentric nuclei, creating a signet ring cell appearance. The colloid was scant. These cytological features may be seen in cases of papillary thyroid carcinoma or signet ring cell follicular adenoma; however, the presence of the signet ring cells is unusual in primary thyroid lesions and raises the possibility of a metastatic lesion to the thyroid. In our case, the tumor cells were positive for AE1/AE3, mucicarmine, and periodic acid-Schiff, but negative for thyroglobulin and thyroid transcription factor-1. The patient was also found to have a 3.7-cm mass in the distal esophagus/proximal stomach. Biopsy of this mass showed an invasive signet ring cell carcinoma. The purpose of our study is to discuss the cytological features and the differential diagnosis of this unusual thyroid FNA case. Diagn.