Twenty-four cases of anaplastic thyroid carcinoma seen in the National Taiwan University Hospital from 1980 to 1988 were reviewed. The median age was 61.5 years. The ratio of men to women was 1:1.4. A preceding history of long-standing goiter could be obtained in 14 patients (58.3%) with a median duration of 20 years. Twenty patients (83.3%) presented with a rapid-growing neck mass with a median duration of 1.3 months. Nine out of 19 patients (47.4%) had white blood cell (WBC) counts over 10,000/microliters, which were closely related to the presence of fever (p less than 0.02). Serial follow-up showed that WBC and platelets gradually increased but red blood cells and hemoglobin decreased. Fever was noted at the time of diagnosis in 8 out of 24 patients (33.3%), and was closely related to the presence of abundant neutrophils in the cytologic smears of thyroid aspirates (p less than 0.01). Fever was not due to bacterial infection. Three out of 10 patients (30.0%) had calcification in the thyroid shown on neck X ray, and 7 out of 18 patients (38.9%) had lung metastasis shown on chest X ray. The ultrasonographic images of thyroid anaplastic carcinoma done in 6 patients were heterogeneous and hypoechoic. Fine-needle aspiration cytology was done in 20 patients. Anaplastic thyroid carcinoma was easily diagnosed in 18 patients (90.0%). One was misdiagnosed as acute thyroiditis. Another one was diagnosed as papillary carcinoma cytologically, which combined with anaplastic carcinoma shown on histology. The median survival was 2.2 months, ranging from 0.2 to 25.3 months.(ABSTRACT TRUNCATED AT 250 WORDS)