Five thyroid carcinomas showing clinically aggressive biologic behavior were retrospectively reviewed to evaluate the possible presence of morphologic differences from conventional thyroid carcinomas. They were originally diagnosed as follicular carcinomas, medullary carcinoma, papillary carcinoma, and mixed papillary and medullary carcinoma. There were three males and two females. The age at the time of initial diagnosis ranged from 36 years to 67 years (mean 56 years). The size of the tumor varied from 4.5cm to 10cm (mean diameter 7cm). One patient died of brain metastasis four years after the initial therapy and the other four patients are still alive with local recurrences and/or metastases to bone, spinal cord, lung, and buttock. Histologically these lesions are categorized into two different groups: insular variant and columnar cell variant. Insular variant was characterized by well-defined nests (insulae) that are composed of small, uniform cells, frequent tumor necrosis, and hyalinization of the stroma. Columnar cell variant was characterized by tall columnar cells with marked nuclear statification. All five cases stained positively for thyroglobulin and negatively for calcitonin. With the above clinical and histopathological findings, we interpreted these lesions as a poorly differentiated carcinoma, biologically in an intermediate position between well differentiated and anaplastic carcinomas. The rapid and often fatal outcome associated with these variants of poorly differentiated carcinoma warrants aggressive treatment at the time of diagnosis.