Parathyroid carcinoma is a rare disorder accounting for 0.5% to 5% of parathyroid neoplasia. Diagnosis of parathyroid carcinoma in fine needle aspiration cytology (FNAC) is difficult because ail characteristic features of parathyroid carcinoma can be recognized in parathyroid adenoma or hyperplasia. Cellular atypism cannot be used for the diagnostic criteria of parathyroid carcinoma as malignancies of most other organs. We experienced two cases of cytologic features of parathyroid carcinoma confirmed by histologic examination. The majority of tumor cells formed large cohesive clusters, although individual tumor cells were also present. The tumor cells displayed rather pleomorphic round to oval nuclei, occasional prominent nucleoli, and distinct cytoplasmic margin. Occasionally karyolysis, anuclear cells, and nonepithelial cell clusters were noted. The histologic findings showed a partially lobulated architecture, with admixture of sheets of chief cells, oxyphil cells, and occasional water clear cells. The tumor infiltrated into the thyroid parenchyme and perithyroidal soft tissue. The electron microscopic study of case 1 disclosed typical findings of parathyroid neoplasm; clusters of secretory chief cells with centrally located round to ovoid nuclei, moderately clumped heterochromatins and one or two nucleoli. The tumor cells showed conspicous interdigitation of contiguous ceil membrane and intercellular microvilli.