Anaplastic thyroid carcinoma is one of the most aggressive of all human malignant diseases. It has an unfavorable prognosis and responsible for most of the mortality and morbidity rates due to thyroid carcinomas. We present a case of incidental anaplastic thyroid carcinoma and discuss the epidemiology, biology, risk factors, prognostic factors of the disease and the approach to treatment, in the light of the current medical literature. The prognosis is much better in cases with incidental carcinoma compared to the classical type and surgical excision of the tumor has a favorable effect on the results. Our case was followed-up for 1.5 years with no evidence of recurrence or metastasis.