Medullary carcinoma of the thyroid gland is a malignant neuroendocrine tumor arising from calcitonin producing-parafollicular cells. The tumor is clinically divided into sporadic and familial form, constituting about 80% and 20%, respectively. Recently, we experienced a case of unilateral and solitary sporadic medullary carcinoma of the left thyroid gland. The patient was a 9 year-old female, who presented with a palpable mass on the anterior lateral neck of 8 months duration without any familial and personal history of neuroendocrine disease. The cytopathologic findings showed spindle cells and plasmacytoid cells in the background of colloid-like materal. The nuclei were eccentrically located, mildly hyperchromatic and pleomorphic, showing speckled chromatin pattern without nuclear inclusion or prominent nucleoli. The cytoplasm was abundant and had a pale granular cyanophilic appearance. No amyloid materal could be identified.