The clinical data of 20 patients with medullary thyroid carcinoma (MTC) treated in Veterans General Hospital (VGH)-Taipei from 1970 to 1991 were reviewed. DNA content of the MTC and postoperative serum calcitonin (CT) were compared to evaluate patients' survival. All three patients with stage IV disease had aneuploid DNA and died within 7 months. The average survival was 4 months. The other 17 patients were all alive at the time of study and the mean follow up was 86 months. Three (3/7) patients had less than total thyroidectomy while only 2 (2/13) of the total thyoidectomized patients needed reoperation because of tumor recurrence. Of the 15 patients with available calcitonin data 11 had persistent post-operative hypercalcitoninemia. Among these 11 patients 5 out of the 6 patients with demonstrable lesions received reoperation; the other five (5/11) without demonstrable lesions were not operated and lived well in spite of persistent hypercalcitoninemia. One in 4 diploid patients and 9 in 12 aneuploid patients had metastatic lesions. No statistical significant correlation was found between the DNA content and survival, metastases or hypercalcitoninemia, although aneuploid MTC tended to be more advanced. We therefore concluded that total thyroidectomy is the treatment of choice for MTC. Persistent postoperative hypercalcitoninemia without clinical demonstrable lesions can be treated conservatively and DNA aneuploidy cannot be used as a reliable indicator of the grade of malignancy for MTC.