Parathyroid tumors may be difficult to distinguish from thyroid follicular lesions, especially when a tumor is nonfunctioning. We report here two cases of asymptomatic parathyroid carcinoma preoperatively misdiagnosed as thyroid follicular lesions, and one case of parathyroid adenoma showing hyperparathyroidism, and review the cytologic features favoring the diagnosis of parathyroid neoplasm. The cytologic findings that are characterized by clean background, monomorphic small cells, cohesive three-dimensional papillary clusters, small tight clusters with scattered naked nuclei, and well-defined clear cytoplasm favor a diagnosis for the parathyroid lesions. Cytologic findings such as macrofollicular structure, presence of colloid and macrophages, and presence of perivacuolar cytoplasmic granules on May-Grunwald-Giemsa stain support a diagnosis of a thyroid follicular lesion. The cytomorphology of parathyroid tumors is so variable that the distinction from a thyroid lesion cannot be based on the presence or absence of a single feature only but on the cytologic features as a whole.