Following operation for medullary thyroid carcinoma, elevated serum calcitonin levels were present in 18 patients who had no clinically evident residual tumor. In eight of these patients, additional operations have been performed to achieve an appropriate thyroidectomy and cervical lymph node dissection. In six of these eight, elevated serum calcitonin concentrations persist. Extensive clinical examinations failed to identify a metastatic focus of carcinoma. Serum calcitonin concentrations have remained relatively stable for up to six years (average, 3.9 years) of observations in 12 patients after initial or only operations performed from one to 13 years (average 7.3 years) previously. This suggests that microscopic metastases of this carcinoma can remain relatively dormant for years. The management of medullary thyroid carcinoma requires initially adequate cervical operations, periodic serum calcitonin measurements postoperatively, reoperation if adequate neck surgery has not been performed and if serum calcitonin levels remain elevated and only periodic reevaluation for other asymptomatic patients with persistently elevated concentrations of serum calcitonin.