We repon the 5 cases of patients with anaplastic carcinoma of thyroid, that consulted at the Hospital Pablo Tobon Uribe. H.P.T.U. between january 1995 and december 2001, with different clinical manifestations and evolution time. The anaplastic carcinoma of Thyroid is one of the most aggressive and fatal neoplasm for the human being. It represents 5 to 14% of all thyroid carcinomas. The Tumor presents a pick of incidence in the seventh decade of the life and the Male to female ratio is 1.3: 1; 20% of the patients have previous history of well-differentiated thyroid neoplasm and 30% of goiter history. Fine-Needle Aspiration Biopsy carries out with the 94.7% of diagnosis of Anaplastic Carcinoma of Thyroid vs. 90% of the cases according to the literature; cito-morphology is highly specific and easy to recognize and it is considered the election method in this carcmoma type. The age of the five patients oscillates between 60 and 70 years. The evolution time of the illness was approximately two months, 3 patients presented previous thyroid pathology and the predominant symptoms were the obstruction of the superior airway, dysphonia and Superior Vena Cava Syndrome; we reach the diagnosis of these patients through Fine-needle Aspiration Biopsy, in two patients the diagnosis was confirmed with the surgical study of the specimen. The treatrnent was: radiotherapy for one patient and surgical for two patients, one of those also received radiotherapy, two patients didn't received treatrnent and the survival time was 4 months.