Poorly differentiated ("insular") carcinoma of the thyroid shares insular, trabecular, and solid histological patterns that are different from those of papillary, follicular, medullary, and anaplastic varieties. This tumor is situated morphologically and biologically in the intermediate position between the well differentiated (papillary and follicular) and the totally undifferentiated (anaplastic) thyroid tumors. We report two cases of insular carcinoma of the thyroid, occurring in 39-year-old and 52-year-old women. Grossly, these cases showed a lobulated mass with fibrous septa. The histologic finding showed characteristic "insular" growth pattern with focal follicular or papillary areas. Thyroglobulin was demonstrated within cytoplasmic paranuclear vacuoles of the neoplastic cells. Calcitonin and amyloid were not demonstrated. The aspiration cytology showed high cellularity, low grade of atypia, presence of clusters, nests, and trabeculae of cells with poorly outlined cytoplasm. The ultrastructural finding showed primordial cells having cytoplasmic organelles such as rough endoplasmic reticulum, mitochondria, and free ribosomes. We believe that its separation from other types of thyroid carcinoma will lead to a more accurate estimate of its biologic behavior and a more appropriate therapeutic approach.