Insular carcinoma of the thyroid is a low differentiated type constituting about 5% of all thyroid cancers. Higher aggressiveness has been suggested as an important clinical feature. The value of preoperative fine-needle aspiration biopsy is not clearly proven for insular carcinoma. The criteria for histological diagnosis have been outlined by Carcangiu et al. Because of its aggressiveness, radical treatment at primary surgery appears advisable. Its clear distinction from undifferentiated (anaplastic) and medullary (C cell) cancers is important, as thyroglobulin is regularly synthesized by cancer cells. Enrichment of radioactive iodine makes such treatment feasible postoperatively and at relapse. Follow-up should be performed as in highly differentiated papillary and follicular thyroid cancer. A patient series of eight cases is presented. While all cancers were advanced at the initial diagnosis, the observed disease courses were in agreement with the assumption that insular carcinoma is a more aggressive form of differentiated thyroid cancer.