Data on four male and four female patients with insular carcinoma of the thyroid were reviewed to elucidate the clinical characteristics of this malignancy. The median age of the patients was 62.5 years (range 29-78 years). The size of the tumors ranged from 3.0 to 10.0 cm (median 4.7 cm). One patient with extensive neck disease and distant metastases underwent palliative debulking of the tumor mass and died of respiratory failure 10 days after surgery. Seven patients underwent total thyroidectomy. In addition to being given thyroxine supplement, three patients were treated with postoperative iodine-131 ((131)I) therapy, two with external radiation after (131)I treatment, and two with external radiation. Local invasion of the malignancy into strap muscles was observed in two patients, into the trachea in two patients, into the recurrent laryngeal nerve in one patient, and into the internal jugular vein in one patient. The median follow-up was 26 months (range 3-80 months). Lung metastases occurred in four patients and metastases at the neck lymph nodes in four patients; recurrence at the thyroid bed was observed in one patient. Four patients died of distant metastases and progressive recurrent disease. Of the three surviving patients, two remained disease-free, and one remained alive with lung metastases. In conclusion, insular carcinomas behave aggressively, so we believe they should be treated by total thyroidectomy plus neck dissection when nodal metastases are present.