With 612 cases of malignant thyroid tumor, the follow-up observations were conducted from 7 to 11 years after initial examination, and 512 cases were follow up. It was found that 46 cases died of the primary cancer, while 13 died of other causes. The prognosis proved to be favorable with adenocarcinoma but with anaplastic carcinoma or squamous cell carcinoma the disease proved to be ultimately fatal. The survival rates 5 and 10 years after diagnosis were 97.4% and 95.8% for papillary adenocarcinoma, 95.6% and 92.5% for follicular adenocarcinoma, 7.7% and 7.7% for anaplastic carcinoma, 0% for squamous cell carcinoma, and 66.9% and 66.9% for malignant lymphoma. As to the causes of death, the obstruction due to the neck tumor ranks first, indicating that respiratory deaths are numerous. However, as there are histologically many deaths due to the transformation from adenocarcinoma to squamous cell carcinoma or anaplastic carcinoma, it is desirable to perform radical surgery at an early stage.